I have a modpack with a zillion mods in it (Well, 156), which runs fine until I add MFR, at which point it fails back to the main menu whenever I try to create or load a world.

It appears to be a problem with MFR's thaumcraft compatibility code, likely caused by one of the several thaumcraft addons I'm using.

Minecraft also hangs when I try to close it when this problem occurs. The window closes but the process doesn't end, and the launcher doesn't come back up.

I've attached the log,

fml-client-latest.log.txt

This is a bug in thaumcraft triggered by one of the mods adding crucible recipes. Duplicate of #446