Update Mekanism.
Is that a known fix or a suggestion/guess? I know its your pack but i've not seen it in the last few threads and others haven't had that problem. Is it going to cause compatibility issues with anything else?
Also i've not seen you mention is there going to be a 1.2.3 coming with these updates to prevent people having to update mods manually as I'm at the point where I'd probably just wait for that and stick on 1.2.0. With a lot of trial and error I probably could get my single player world working but trying to talk the people on my server how to manually update what would be trickier.