1.12.2 modded server crashes after a while of running

My issue is that my server crashes 2-3 a day and I can’t figure out how to solve it 

Here is the main portion of the crash log:

---- Minecraft Crash Report ----
// You're mean.

Time: 2021-06-19 07:29:47 HDT
Description: Exception ticking world

java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
    at java.util.HashMap$KeyIterator.next(HashMap.java:1469)
    at net.minecraft.entity.EntityTracker.sendLeashedEntitiesInChunk(EntityTracker.java:386)
    at net.minecraft.server.management.PlayerChunkMapEntry.sendToPlayers(PlayerChunkMapEntry.java:162)
    at net.minecraft.server.management.PlayerChunkMap.tick(SourceFile:165)
    at net.minecraft.world.WorldServer.tick(WorldServer.java:227)
    at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:756)
    at net.minecraft.server.dedicated.DedicatedServer.updateTimeLightAndEntities(DedicatedServer.java:397)
    at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:668)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526)
    at java.lang.Thread.run(Thread.java:748)
 

I think the issue lies within the line “net.minecraft.entity.EntityTracker.sendLeashedEntitiesInChunk(EntityTracker.java:386)” 

but I have no idea what leashedentities could be, would it be any mob or entity that has a leash on it?