Random crash (1.12.2)

Hi! Maybe you can help. the server closes at random moments. I’m desperate
I am ready to answer any clarifying questions!
logs just before the crash:

net.minecraft.server.v1_12_R1.ReportedException: Exception ticking world
at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:916) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:423) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:774) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:666) ~[patched_1.12.2.jar:git-Paper-1618]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_221]
Caused by: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445) ~[?:1.8.0_221]
at java.util.HashMap$KeyIterator.next(HashMap.java:1469) ~[?:1.8.0_221]
at net.minecraft.server.v1_12_R1.PlayerChunkMap.flush(PlayerChunkMap.java:126) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.WorldServer.doTick(WorldServer.java:332) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:905) ~[patched_1.12.2.jar:git-Paper-1618]
… 4 more
[12:03:30] [Server thread/ERROR]: Cause of unexpected exception was
java.util.ConcurrentModificationException: null
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445) ~[?:1.8.0_221]
at java.util.HashMap$KeyIterator.next(HashMap.java:1469) ~[?:1.8.0_221]
at net.minecraft.server.v1_12_R1.PlayerChunkMap.flush(PlayerChunkMap.java:126) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.WorldServer.doTick(WorldServer.java:332) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:905) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:423) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:774) ~[patched_1.12.2.jar:git-Paper-1618]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:666) ~[patched_1.12.2.jar:git-Paper-1618]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_221]
[12:03:30] [Server thread/ERROR]: This crash report has been saved to: /home/srv37827/./crash-reports/crash-2021-10-13_12.03.30-server.txt
[12:03:30] [Server thread/INFO]: Stopping server

Versions prior to 1.17.1 are unsupported.

not supported for helping? or not supported by paper?

ok. thanks anyway