I was playing in a LAN server with a friend, using Hamachi to connect, and checking out the new snapshot, 15w31a. We went to the End and went through the portal to the place with the huge purple flowers and such. I tried to place down a sign and it kicked me out with an error message. I tried to join back and every time I did it gave me the same error message. It wasn't until my friend destroyed the sign I placed that I could join. While typing this message, my friends tried other things with signs. According to them, placing any signs at all results in the same issue. Whether in the regular world or the End. I have included a screen capture of the message, that all of us got when placing a sign, as an attachment.