After a very long while, I found a solution to the first of two problems described in
this post. I've had the same troubles for some time and could not think of the reason until today, I switched the kde desktop effects off and through this disabled the composite manager. Lo and behold, the intro movies are playing fine again.
The second problem has been present for years already and seems impossible to solve. If nwn runs in windowed mode, every movie is played fine, but in full screen, the movies are played but the display does not show them - you still hear the sound, though.
Greetings
Skildron