Hnyakwai

I don't know your particular problem, but I found that the best way for debugging things like this is to set all the debug options in TobEx to 1 (especially "More Messages"), and then check out the TobEx.log where you crashed. Then if you can find exactly what bcs was running when it tried to load something illegal it makes the issue much easier to track.

Is there a similar problem with SET_BG2_PROFICIENCY, since it also adds effects?

I don't think so. The problems I identified were the results of alterations made to Gort's macros in WeiDU 221. SET_BG2_PROFICIENCY is completely separate (written in OCaml, even) as far as I know.

Hnyakwai, which version of WeiDU did you use to install aTweaks (it should say at the top of the aTweaks debug file)?
WeiDU 221 was released pretty recently and I want to be sure the problems I found are not a different from the problems you are having (though that would be pretty unlikely).

I was using WeiDU 221.  After I reported this I did a completely clean install (and since I was tired of debugging after 48 hours or so of constant struggle, I disabled anything that I thought could possibly corrupt my install... which means no aTweaks this time)  I dug up the files from the BWS backups though.  Your mod looks like it has a lot of great features, so I would definitely like to install it next time I reinstall.

Um, I suppose I should ask which codepage your system is using. Because if not a Western European one, then Infinity Animations could be causing it, but it sounds like you traced it to something from before you installed that.

What do you mean by codepage?  I'm in North America so I guess I probably don't have a Western European codepage.  I think I run about 10 BWP installs this weekend and I have been rolling back with BWS setup.  I think it is time for me to wipe everything else and try from scratch, do I need to disable IA with a North American computer?