My server crash sometimes

I have a modded server with 6-10 players, 4gb RAM and 2 vCores (AMD Ryzen 3900X 4.6GHz). Sometimes crash and I can't know what happens.

Here the error:

[16:06:26] [Server thread/ERROR] [net.minecraft.server.MinecraftServer]: Encountered an unexpected exception
net.minecraft.util.ReportedException: Exception ticking world
    at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:762) ~[MinecraftServer.class:?]
    at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397) ~[nz.class:?]
    at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668) ~[MinecraftServer.class:?]
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
    at java.lang.Thread.run(Thread.java:748) [?:1.8.0_252]
Caused by: java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445) ~[?:1.8.0_252]
    at java.util.HashMap$KeyIterator.next(HashMap.java:1469) ~[?:1.8.0_252]
    at net.minecraft.entity.EntityTracker.func_85172_a(EntityTracker.java:386) ~[ol.class:?]
    at net.minecraft.server.management.PlayerChunkMapEntry.func_187272_b(PlayerChunkMapEntry.java:162) ~[ot.class:?]
    at net.minecraft.server.management.PlayerChunkMap.func_72693_b(SourceFile:165) ~[ou.class:?]
    at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:227) ~[oo.class:?]
    at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:756) ~[MinecraftServer.class:?]
    ... 4 more
[16:06:26] [Server thread/ERROR] [net.minecraft.server.MinecraftServer]: This crash report has been saved to: /home/minecraft/servers/mine285700/./crash-reports/crash-2020-12-07_16.06.26-server.txt
[16:06:26] [Server thread/INFO] [net.minecraft.server.MinecraftServer]: Stopping server
[16:06:26] [Server thread/INFO] [net.minecraft.server.MinecraftServer]: Saving players
[16:06:26] [Server thread/INFO] [net.minecraft.server.MinecraftServer]: Saving worlds
[16:06:26] [Server thread/INFO] [net.minecraft.server.MinecraftServer]: Saving chunks for level 'world'/overworld
[16:06:27] [Server thread/INFO] [THAUMCRAFT]: Removing aura cache for world 0
[16:06:27] [Server thread/WARN] [mca]: Crash detected! Attempting to upload report...
[16:06:27] [Server thread/ERROR] [mca]: Failed to submit crash report. Non-OK response code returned: 301
[16:06:27] [Thread-8/WARN] [THAUMCRAFT]: No auras found!
[16:06:27] [Thread-8/INFO] [THAUMCRAFT]: Stopping aura thread for dim 0
[16:06:29] [Server thread/INFO] [FML]: Unloading dimension 0
[16:06:29] [Server thread/INFO] [FML]: The state engine was in incorrect state SERVER_STOPPING and forced into state SERVER_STOPPED. Errors may have been discarded.
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.server.MinecraftServer]: Stopping server
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.server.MinecraftServer]: Saving players
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.network.NetHandlerPlayServer]: Althruis lost connection: Server closed
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.server.dedicated.DedicatedServer]: Althruis left the game
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.network.NetHandlerPlayServer]: WebosRotos3004 lost connection: Server closed
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.server.dedicated.DedicatedServer]: WebosRotos3004 left the game
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.network.NetHandlerPlayServer]: MdavidM9 lost connection: Server closed
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.server.dedicated.DedicatedServer]: MdavidM9 left the game
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.network.NetHandlerPlayServer]: Rokordo lost connection: Server closed
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.server.dedicated.DedicatedServer]: Rokordo left the game
[16:06:29] [Server Shutdown Thread/INFO] [net.minecraft.server.MinecraftServer]: Saving worlds
 

Sorry for my bad English, if anyone can help me pls