When placing down a sign all online (at least those within maybe 30 blocks of the sign) clients crash. The message the clients receive are as follows: "...gson.MalformedJsonException... JSON at line 1 column 2"(http://supa.me/EAOxOU.png).
The console spams the following error (https://gist.github.com/SupaHam/e024d2be25622576a33c).
P.S. Restarting the server does not fix anything.
For what it's worth. Getting kicked in general displays that client error. so I'm assuming the actual bug causing the crashing is not something I can track or help you track down on my end.