Can't Connect to Server

When I try to connect to a 15w40a server:
Client: gets kicked with message:
"Internal Exception: io.netty.codec.DecoderException: java.lang.IndexOutOfBoundsException:readerIndex(9) + length(1) exceeds writerIndex(9):UnpooledHeapByteBuf(ridx: 9, widx: 9,cap: 9)"

Server consol gets spamed with 2-10 (depending on the world) messages:
"Netty Epoll Server IO #4/ERROR: java.lang.ClassCastException: io.netty.buffer.UnpooledUnsafeDirectByteBuf cannot be cast to em"

We tested this on 2 servers.
Sometimes this also happend on "15w39c" but not on connect and only if there were to much commandblocks on the server.