Hi Sebastian - yeah the folds that Burr shows above will cause numerous problems with surface/surface intersection mechanisms - many operations such as trimming, booleans, or filleting will not usually work right on surfaces that are messed up in that particular way.

I don't see any of the original generator curves used to create that surface still in this file - you'll probably want to check those curves out and make sure that the original curves that were being used don't have a folded over area in the curve - if it does then usually surfaces created from that kind of curve will inherit the same problem as well.

- Michael