I'm not sure if this is actually Botania's fault or not, but it's the only mod in the stacktrace. Someone was fighting a Gaia 2 with an Enderium Rapier (in the mob spawning phase) when the server crashed and was thrown into limbo (I had to manually stop then start the server; normally it restarts automagically). Upon restart it works fine. Here is the generated crash report: http://pastebin.com/DXV9VUB3

This seems like a bug in one of the CoFH mods.