ProjectRed version: ProjectRed-1.12.2-4.9.0.86-Base
MrTJPCore version: MrTJPCore-1.12.2-2.1.2.34-universal

Steps to reproduce:

place down a frame motor
power it
activate it to move a block or fame
Client will disconnect immediately and the server will show the following in the console. Server doesn't crash though, client can reconnect immediately. the blocks move after reconnecting

[14:48:57] [Server thread/INFO] [net.minecraft.network.NetHandlerPlayServer]: SpeedrunnerG55 lost connection: Internal Exception: io.netty.channel.unix.Errors$NativeIoEx ception: syscall:read(..) failed: Connection reset by peer
and the disconnect message is

project red internal exception io.netty.handler.coder.decoderexception: the received string is longer then the maximum allowed (21 > 20)

You want to update before asking for help.