Okay, thanks guys. The 5-sided faces were the problem. I could fix some of them but I'm stuck at several faces. If I try to make them 4-sided by merging some vertecies or trying to fix them via split polygon tool, there appear new 5 or 3 sided faces in the area around the face I just fixed. This happens on and on, these 3 and 5 sided faces are like a chainreaction on my model. I fix one, another appears, I fix the next one, another appears... this is making me crazy. I'd appreciate any hints or help. "Cleaning" up a polygon model is really tricky.