Here are several of the errors I get when using Realms:
Internal Exception: io.netty.handler.codec.DecoderException: java.util.zip.DataFormatException: incorrect header check
Internal Exception: io.netty.handler.codec.DecoderException: Badly compressed packet - size of 1531896730 is larger than protocol maximum of 2097152
Internal Exception: io.netty.handler.codec.CorruptedFrameException: length wider than 21-bit
Internal Exception: io.netty.handler.codec.DecoderException: java.util.zip.DataFormatException: incorrect header check
Internal Exception: io.netty.handler.codec.CorruptedFrameException: length wider than 21-bit (I know, this one repeated, they do that too)
Internal Exception: io.netty.handler.codec.DecoderException: Badly compressed packet - size of 11478809 is larger than protocal maximum of 2097152
Internal Exception: io.netty.handler.codec.DecoderException: Badly compressed packet - size of 102 is below server threshold of 256
attatched is my crash report
This appears to be a duplicate of REALMS-1656, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.