Updated FML:
MinecraftForge/FML@913f6f6d36bd179db7c147db0485e99dee693933 Try and use the relaunch log, which should be classloaded..
MinecraftForge/FML@ac065ff5f76b6c512b346366107efde66e9e1c88 Reset the IWorldGenerator seed for each mod, before calling. That should mean worldgen is consistent and not dependent on mod ordering, or mod sideeffects.