I'm having the exact same problem. In mine it says:
“Internal Exception: io.netty.handler.codec.DecoderException: java.io.IOException: Packet 0/26 (pn) was larger than I expected, found 166 bytes extra whilst reading packet 26“
My username is definitely correct without any spaces and 11 letters and I'm playing on vanilla 1.16.3 with launcher Tlauncher