We do use sponge on the server and it is mentioned in the crash report.

http://pastebin.com/VCPRTkWV

Seems like a bad interaction with a vanilla hopper as reported by a player.

I can't (and don't want to) fix bugs that only happen when sponge is installed. If you can reproduce this on a normal forge server that would be nice :).

Well thats just silly. Disregard than.