http://pastebin.com/HyNtLXVE there's the crash log, it seems similar to a crash that was occurring in an earlier 1.6 version. Hope you can get it fixed.

It's upon a mob-spawner, I don't know if it's an incompatibility thing, but it's showing up in my pack, With almost the same exact log. The world/server can not boot when this happens. Removing the mod fixes it, I'll be doing more testing tomorrow to see if I can find a more safe-way of fixing it.