Exception in server tick loop

Please, I need help...

>>> My FML log: https://pastebin.com/raw/KueNC4Hr

---- Minecraft Crash Report ----

WARNING: coremods are present:
  llibrary (llibrary-core-1.0.11-1.12.2.jar)
  CorePlugin (AcademyCraft-1.1.2.jar)
  CreativePatchingLoader (CreativeCore_v1.10.13_mc1.12.2.jar)
  CTMCorePlugin (CTM-MC1.12.2-1.0.2.31.jar)
Contact their authors BEFORE contacting forge

// This is a token for 1 free hug. Redeem at your nearest Mojangsta: [~~HUG~~]

Time: 5/20/20 11:43 PM
Description: Exception in server tick loop

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.func_72788_a(EntityTracker.java:291)
    at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:926)
    at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:472)
    at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:782)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:636)
    at java.lang.Thread.run(Thread.java:748)

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