Hi Heiner, also in addition to what Burr wrote above it would probably be a good idea to construct the top flat side as one long piece rather than having it split up into many smaller sections. N-sided treats each each side with an equal weight basically and when they are out of proportion to each other like some much smaller than others it can make for bad results.

I've attached a version here with that back side reconstructed as one large piece using Network. Then the surrounding edges can be selected (as written by others above you need to select _edge_ objects for input to N-sided, if you have regular curves that won't work), and tone down the bulge a bit to get this kind of result.

- Michael