Mm, you're right, I guess I'll have to do more research on the assertion failure in King Strohm's tomb. It seems to be triggered by the guardian's appearance as I bypassed it by letting that happen off screen.
All other instances of the failure were triggered by d0salfir/d0salice though, all these areas have creatures with this effect and I can reliably trigger the bug by spawning icsalfir with the console, which immediately crashes my game as soon as the flame shield comes up. And well, I suppose there is a reason DLTCEP issues that warning...
Game version is BG2 complete from GoG which has all official patches included, and ToBex 26beta installed.