I was building a T4 shield and when I turned it on with 6k+ blocks, I was kick with
Internal Exception: io.netty.handler.codec.DecoderException: java.IOException: Packet was larger than I expected,found 41939 bytes extra whilst reading packet 53

Was this on a server?

Yes it was. It was with rftools-4.0.1

This will probably be fixed in next version