Minecraft server crashes when loading new chunks?

Question

Java

---- Minecraft Crash Report ----
// I just don't know what went wrong :(

Time: 7/28/20 4:28 PM
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.server.v1_12_R1.PlayerChunkMap.flush(PlayerChunkMap.java:122)
at net .minecraft.server.v1_12_R1.WorldServer.doTick(WorldServer.java:320)
at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:801)
at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer .java:406)
at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679)
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577)
at java.lang.Thread.run(Thread.java:748)