Crash directly at start of the archdemon boss-fight in Weisshaupt

In my first playthrough I breezed through the game, no crashes/bugs/whatsoever.

But in my second playthrough (same gamesettings), the game crashed just at the beginning of the archdemon/boss fight directly after trapping the dragon in the dragontrap at Weisshaupt. The autosave just before that crash can't be loaded either, since it crashes during loading. If I load a game way before the crash, it still crashes when I come to that fight.

I started again, and at my third playthrough, the game crashes at exactly the same spot.

I don't understand why I had no trouble at all during the first playthrough.

Does anyone else have this problem? And hopefully, does anyone have a solution?