Title says it all, here's the crash report: http://paste.ubuntu.com/23058225/

EDIT:
Wow, the Questioner is basically a ticking time bomb. If there's multiple entries in it and any of them time out, it will likely crash the server. See https://github.com/ForgeEssentials/ForgeEssentials/blob/develop/src/main/java/com/forgeessentials/util/questioner/Questioner.java#L109

This needs special handling rather than calling cancel(), as you need to use an Iterator and call remove() to prevent this crash.

Apparently this was first reported back in October 2015 (#1820). Why was this never fixed? I have no idea, but there, I fixed it for you.