Placing signs

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.