Unknown player join server crashed and restarted

I use Waterfall to connect multiple servers in series, and the server that often crashes (login lobby server) has the same plug-in version as other servers, but it usually crashes when the player logs in. This problem does not occur with other servers.

crash-reports:

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(Unknown Source)
	at java.util.HashMap$KeyIterator.next(Unknown Source)
	at net.minecraft.server.v1_16_R2.PacketPlayOutUpdateAttributes.<init>(SourceFile:27)

You have a plugin on your server that is messing with attributes, I’d suggest doing the typical “start ripping out plugins”, and maybe update to a supported version of paper

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