Problem/Bug Internal Exception: Unknown serializer type 114, 97, 99, 111, etc

The players on my server are getting the "Unknown serializer type" error when they try to login. Sometimes it only takes a few tries, sometimes loading single player first helps, sometimes restarting the server helps and sometimes nothing helps. Anyone know what's causing it?

Server specs:
Java with 16GB ram allocated (64GB server computer)
Arclight forge - 1.0.24
Running minecraft 1.16.5
Pixelmon: 9.1.2
LuckyPerms - Bukkit - 5.4.40
Multiverse core 4.3.0
Multiverse inv 4.2.3
Multiverse netherportals 4.2.2
multiverse portals 4.2.1
permission bukkit - 2.5
essentials chat
teleport request

Not sure how to post a log, so here is the end of it:

[10:57:03] [main/INFO]: Created: 2048x2048x0 minecraft:textures/atlas/particles.png-atlas
[10:57:03] [main/INFO]: [net.minecraft.client.renderer.texture.AtlasTexture:handler$zfa000$transparent_reload:1021]: minecraft:textures/atlas/particles.png
[10:57:03] [main/INFO]: Created: 16384x8192x0 minecraft:textures/atlas/paintings.png-atlas
[10:57:03] [main/INFO]: [net.minecraft.client.renderer.texture.AtlasTexture:handler$zfa000$transparent_reload:1021]: minecraft:textures/atlas/paintings.png
[10:57:03] [main/INFO]: Created: 2048x2048x0 minecraft:textures/atlas/mob_effects.png-atlas
[10:57:03] [main/INFO]: [net.minecraft.client.renderer.texture.AtlasTexture:handler$zfa000$transparent_reload:1021]: minecraft:textures/atlas/mob_effects.png
[10:57:06] [main/INFO]: [FANCYMENU] Updating animation sizes..
[10:57:06] [main/INFO]: Reloading sounds!
[10:57:06] [main/INFO]: Running post-reload tasks..
[10:57:56] [main/INFO]: Connecting to 192.86.0.208, 25565
[10:57:58] [main/INFO]: Injecting existing registry data into this CLIENT instance
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:acacia_twiglet_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:burnt_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:cherry_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:cypress_willow_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:dark_oak_twiglet_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:dark_poplar_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:dark_poplar_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:dead_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:dying_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:dying_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:fir_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:fir_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:fir_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:flowering_oak_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:flowering_oak_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:hellbark_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:hellbark_bush_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:hellbark_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:hellbark_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:jacaranda_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:jacaranda_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:jacaranda_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:jungle_twiglet_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:magic_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:magic_poplar_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:magic_poplar_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:mahogany_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:mahogany_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:mahogany_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:maple_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:maple_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:maple_twiglet_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:mega_dark_oak_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopak_twiglet_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtboprange_autumn_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtboprange_autumn_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopalm_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopalm_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopalm_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopink_cherry_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopink_cherry_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopoplar_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbopoplar_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rainbow_birch_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rainbow_birch_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:redwood_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:redwood_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:redwood_root
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:redwood_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rooty_black_sand
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rooty_dried_salt
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rooty_mud
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rooty_orange_sand
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rooty_origin_grass_block
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:rooty_white_sand
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:silk_web
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:sparse_acacia_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:sparse_oak_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:spruce_poplar_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:spruce_poplar_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:spruce_twiglet_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_cherry_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_dead_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_fir_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_hellbark_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_jacaranda_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_magic_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_mahogany_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_palm_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_redwood_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_umbran_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:stripped_willow_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:umbran_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:umbran_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:umbran_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:white_cherry_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:white_cherry_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:willow_branch
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:willow_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:willow_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:yellow_autumn_leaves
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world dtbop:yellow_autumn_sapling
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermal:crossover_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermal:lumium_activator_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermal:lumium_crossover_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermal:lumium_detector_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermal:lumium_powered_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermal:lumium_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermalrismarine_activator_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermalrismarine_crossover_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermalrismarine_detector_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermalrismarine_powered_rail
[10:57:58] [main/INFO]: Registry minecraft:block: Found a missing id from the world thermalrismarine_rail
[10:58:09] [Netty Client IO #1/INFO]: SERVER CONFIG LOADED
[10:58:09] [Netty Client IO #1/INFO]: Loading JourneyMap Forge Configs
[10:58:09] [Netty Client IO #1/INFO]: Connected to a modded server.
[10:58:10] [main/INFO]: Started 10 worker threads
[10:58:10] [main/INFO]: Environment: authHost='https://authserver.mojang.com', accountsHost='https://api.mojang.com', sessionHost='https://sessionserver.mojang.com', servicesHost='https://api.minecraftservices.com', name='PROD'
[10:58:10] [main/INFO]: [CHAT] §a[Extra Storage] §fA new version is available (null), please update!
[10:58:10] [main/INFO]: [CHAT] §eChangelog:
[10:58:10] [main/FATAL]: Error executing task on Client
java.util.NoSuchElementException: packet_handler
at io.netty.channel.DefaultChannelPipeline.getContextOrDie(DefaultChannelPipeline.java:1119) ~[netty-all-4.1.25.Final.jar:4.1.25.Final]
at io.netty.channel.DefaultChannelPipeline.addBefore(DefaultChannelPipeline.java:260) ~[netty-all-4.1.25.Final.jar:4.1.25.Final]
at io.netty.channel.DefaultChannelPipeline.addBefore(DefaultChannelPipeline.java:249) ~[netty-all-4.1.25.Final.jar:4.1.25.Final]
at net.minecraftforge.network.NetworkFilters.lambda$injectIfNecessary$1(NetworkFilters.java:48) ~[?:?]
at net.minecraftforge.network.NetworkFilters$$Lambda$21574/135654690.accept(Unknown Source) ~[?:?]
at com.google.common.collect.RegularImmutableMap.forEach(RegularImmutableMap.java:146) ~[guava-21.0.jar:?]
at net.minecraftforge.network.NetworkFilters.injectIfNecessary(NetworkFilters.java:44) ~[?:?]
at net.minecraftforge.fml.network.NetworkHooks.sendMCRegistryPackets(NetworkHooks.java:121) ~[?:?]
at net.minecraft.client.network.play.ClientPlayNetHandler.func_147282_a(ClientPlayNetHandler.java:399) ~[?:?]
at net.minecraft.network.play.server.SJoinGamePacket.func_148833_a(SourceFile:110) ~[?:?]
at net.minecraft.network.play.server.SJoinGamePacket.func_148833_a(SourceFile:18) ~[?:?]
at net.minecraft.network.PacketThreadUtil.func_225383_a(SourceFile:21) ~[?:?]
at net.minecraft.network.PacketThreadUtil$$Lambda$21370/963947927.run(Unknown Source) ~[?:?]
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(SourceFile:144) [?:?]
at net.minecraft.util.concurrent.RecursiveEventLoop.func_213166_h(SourceFile:23) [?:?]
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(SourceFile:118) [?:?]
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213160_bf(SourceFile:103) [?:?]
at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:948) [?:?]
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:607) [?:?]
at net.minecraft.client.main.Main.main(Main.java:184) [?:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51]
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51]
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:51) [forge-1.16.5-36.2.34.jar:36.2]
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$493/1444500451.call(Unknown Source) [forge-1.16.5-36.2.34.jar:36.2]
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.1.3.jar:?]
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.1.3.jar:?]
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.1.3.jar:?]
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.1.3.jar:?]
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.1.3.jar:?]
[10:58:21] [main/INFO]: Stopping worker threads
[10:58:28] [main/INFO]: Stopping!