After joining a singleplayer world and quitting to menu, Minecraft freezes. Opening a world for the first time and using the main menu before joining a world works fine. Subsequent "first joins" work fine as well. I have confirmed multiple times that it is indeed the base Mekanism mod that is causing the freeze or an interactition between Mekanism and another mod. When I take Mekanism out of the mod file, the freeze goes away. Adding or removing the generators or tools addons does nothing. I am running just the base Mekanism, Forge 1424 and Liteloader_04. I've tried Mekanism versions 231-233 to no avail.

Forge log
There is no crashlog - this isn't a crash

Working fine with exclusively Mekanism and IC2 in my dev environment. I'd call this a corner case, do let me know if you find the offending mod though.