First -- I do love the mod, so please take this as bug-detecting and user-feedback, not criticism.
The initial error was that, when the PC (using Priest of Thieron kit) leveled, she got insulted by something, then got her "new" spells (same as before) and all was well and happy - so, like in Unix, we go on since it ws not a fatal error. However, at Garren's cabin (in Windspear Hills, AR1204) after the child extraction quest, when Garren gives the XP, the game would have this assertion error: an assertion failed in CVisibility.cpp at line number 258. This was a fatal error.
CVisibility led me to think it might be trying to display something and couldn't find the file.
Work around process led me to use Near Infinity to edit the character file, changing the PC's kit out. When the kit was removed and PC turned into a plain cleric, the problem went away.
So, to test this theory, I uninstalled the Thieron kit, then went back to the character file, turned her into Layan Riyashial Priest, went back to the save and the modified character, and look ma! No crashes. (And because I actually do like the kit, I'll be turning her back into a Thieron after this next session.)
If you'd like any of my logs, I'd be happy to provide them.