I am running a vanilla java server in 1.21, and every so often this error will appear followed by additional information:
[Netty Server IO #9/ERROR]: Error sending packet clientbound/minecraft:disconnect
io.netty.handler.codec.EncoderException: Sending unknown packet 'clientbound/minecraft:disconnect'
This began after updating the server.jar file to 1.21. This is not related to whitelisted players joining/leaving as they have had no issues. I suspect this error is thrown when a non-whitelisted or banned player attempts to connect.
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We're tracking this issue in MC-271325, so this ticket is being resolved and linked as a duplicate.
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