Minecraft Error: Internal Exception io.netty.handler.codec.DecoderException: Failed to decode packet 'clientbound/minecraft:game_profile'

I have a problem when trying to join Aternos servers. The server is on version 1.21, but it tells me to try on version 1.20.4, and then it tells me to try on version 1.21 again. I don't know what to do to fix the issue." (This only happens with Aternos servers; I can access other servers without any issue).