SEVERE java.lang.OutOfMemoryError: unable to create new native thread

My server randomly comes up with this error

Code:

18.02 20:39:19 [Server] INFO    at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:435)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:537)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:100)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.NetworkManager.b(NetworkManager.java:226)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.Packet102WindowClick.handle(SourceFile:28)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:983)
18.02 20:39:19 [Server] INFO    at net.minecraft.server.Container.a(SourceFile:128)
18.02 20:39:19 [Server] INFO    at java.util.ArrayList.get(ArrayList.java:382)
18.02 20:39:19 [Server] INFO    at java.util.ArrayList.rangeCheck(ArrayList.java:604)
18.02 20:39:19 [Server] INFO java.lang.IndexOutOfBoundsException: Index: 77, Size: 45
18.02 20:39:19 [Server] WARNING Failed to handle packet: java.lang.IndexOutOfBoundsException: Index: 77, Size: 45
[Server] SEVERE    at java.lang.Thread.run(Thread.java:722)
[Server] SEVERE    at org.bukkit.craftbukkit.scheduler.CraftScheduler.run(CraftScheduler.java:64)
[Server] SEVERE    at org.bukkit.craftbukkit.scheduler.CraftScheduler.processTask(CraftScheduler.java:107)
[Server] SEVERE    at org.bukkit.craftbukkit.scheduler.CraftThreadManager.executeTask(CraftThreadManager.java:14)
[Server] SEVERE    at org.bukkit.craftbukkit.scheduler.CraftWorker.<init>(CraftWorker.java:28)
[Server] SEVERE    at java.lang.Thread.start(Thread.java:691)
[Server] SEVERE    at java.lang.Thread.start0(Native Method)
[Server] SEVERE java.lang.OutOfMemoryError: unable to create new native thread
18.02 20:39:17 [Server] SEVERE Exception in thread "Thread-4" 

This happens randomly for no reason. I have a VPS with 4gb of ram with an average of 30 players online. It seems to me as a memory leak but I don't know what is causing the problem. The problem is a do have a good amount of free memory so I don't know why my server is giving out this error. After this error is processed about 85% of my plugins stop working.

Plugins:
Anticreeper
AntiPub
Automessage
Buttonwarps
chestshop
dropclear
essentials
essentialsspawn
falsebook
hawkeye
herochat
lockette
iConomy
minequery
Mobeggs
modifyworld
Multiverse-Core
No Cheat
Permissions
PermissionsEX
SetRankPEX
SimpleHat
Vault
Votifier
WhoMode
World Edit
World Guard
World Inventories

Bukkit Build: 1.1-R4
All Plugins are updated to most recent builds those plugins are updated for.
We have 7 Worlds running on the server.
I understand this might all seem like the server is overloaded, but we don't run out of mem.
We have atleast 2gb free I don't know what's wrong.

 

How much RAM is allocated to your Minecraft?

Try updating Java.

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

Me too :/