Hi bisenberger, it's most likely having some difficulty with this juncture area here:




You probably don't want to have this other edge here at all:




It's probably having some problems trying to form a good corner patch in that area while trying at the same time to deal with the presence of that other edge - you probably want that piece to be one single larger surface and not have an edge there and that will simplify the corner condition for filleting.

- Michael