I had a
very similar problem with the DOS version. I never found a solution (and gave up, because I didn't want to start over either).
This sucks.
ETA: It's weird that this bug seems completely unknown, even though the dozens of bugs in QfG4 have been well documented for years. I'm starting to wonder if it's not a side effect of the unofficial patches... It'd be interesting to try it with an unmodified version of the game and see if it still occurs.