Problem/Bug Server keeps crashing

Hey, my MC bukkit 1.11.2 server keeps randomly crashing- and staying closed. When I'm offline I can't restart it and therefore this issue needs to be fixed.
It happened twice- here are the crash logs for one:

Crashlog (open) Crashlog (close)

---- Minecraft Crash Report ----
// Oops.

Time: 4/21/17 1:54 PM
Description: Exception ticking world

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at net.minecraft.server.v1_11_R1.PlayerChunkMap$3.a(PlayerChunkMap.java:71)
at net.minecraft.server.v1_11_R1.PlayerChunkMap$3.computeNext(PlayerChunkMap.java:98)
at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:143)
at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:138)
at net.minecraft.server.v1_11_R1.WorldServer.j(WorldServer.java:398)
at net.minecraft.server.v1_11_R1.WorldServer.doTick(WorldServer.java:248)
at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:739)
at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:360)
at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:650)
at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:554)
at java.lang.Thread.run(Unknown Source)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Server thread
Stacktrace:
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at net.minecraft.server.v1_11_R1.PlayerChunkMap$3.a(PlayerChunkMap.java:71)
at net.minecraft.server.v1_11_R1.PlayerChunkMap$3.computeNext(PlayerChunkMap.java:98)
at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:143)
at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:138)
at net.minecraft.server.v1_11_R1.WorldServer.j(WorldServer.java:398)
at net.minecraft.server.v1_11_R1.WorldServer.doTick(WorldServer.java:248)

-- Affected level --
Details:
Level name: world_nether
All players: 0 total; []
Chunk stats: ServerChunkCache: 4108 Drop: 84
Level seed: -8073284899744189576
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (0,81,0), Chunk: (at 0,5,0 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 28616667 game time, 28616667 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:739)
at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:360)
at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:650)
at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:554)
at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
Minecraft Version: 1.11.2
Operating System: Linux (amd64) version 2.6.32-642.6.2.el6.x86_64
Java Version: 1.8.0_111, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 589730248 bytes (562 MB) / 2075918336 bytes (1979 MB) up to 2075918336 bytes (1979 MB)
JVM Flags: 5 total; -Xmx2048M -Xms2048M -XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled -XX:MaxMetaspaceSize=128M
IntCache: cache: 2, tcache: 0, allocated: 12, tallocated: 94
CraftBukkit Information:
Running: CraftBukkit version git-Bukkit-41a7d14 (MC: 1.11.2) (Implementing API version 1.11.2-R0.1-SNAPSHOT) true
Plugins:
[Removed rest of error log to save space]