Fall-From-Grace Level Up Glitch

[SpectralTime]

Hello. I'm playing this game for the first time, and having a blast. However, I've run into a problem.

I've only just gotten Fall-From-Grace and Ignus into my party, and Fall-From-Grace is ready to Level Up for the first time. However, whenever I click the Level Up button on her stat screen, regardless of where I am or who's in the party, the game crashes and I get the following error message:

An assertion failed in F:\Torment\Source\baldur\CRuleTables.cpp at line number 1327.

Help!

For further information, I'm playing on a Dell Inspiron 1525, with the official patch, a full install, and the following mods, listed in the order of installation:

The Bigg's Widescreen Mod
Ghostdog's User Interface Mod
Qwinn's Fixpack, Unfinished Business, and Tweaks Mods (in that order)

I pretty much followed this guide doing it.
Useful Files & MODs for Planescape: Torment - Bootstrike.Com

What does the message mean, and how can I fix it?