After leaving my server for a few hours I rejoined and the server crashed.

Crash Report: http://pastebin.com/4ixEsfgZ

Some core mod is patching net.minecraft.world.World.isOutsideBuildHeight(BlockPos) in a bad way. Report this crash with that mod. You logfile may tell you which one is patching it (most of them announce it when they do), if not, report this to all listed core mods that do not write a notice about patching something into the log.

not sure which one it is, so here is the log: https://drive.google.com/file/d/0BxQWn9jKBIAJYm53SlllQjVmNTQ/view?usp=sharing