In your mod testing forum Neovalen posted this:
"Despite the fact that it's recommended in the CK bug fixes area here, for some reason the ini tweak iLargeIntRefCount=999999 causes Helgen Keep to crash my game. I haven't found any other areas specifically bugged by this tweak, there is probably an epic bug of some type in the worldspace there.
Removing the tweak allowed Helgen Keep to load fine. Just putting this out there in case anyone else runs into it. I did put the tweak back in after exiting Helgen."
I'd just like to comment that I ran through the vanilla start with that tweak and did not have any problems with it (or anything else for that matter ;) ), so its probably not something caused by the tweak itself. I did not, however, have a full STEP install at the time.