Andreas,
What kind of boat? I have seen this problem frequently on the Nacra 6.0 NA and it is usually mis-diagnosed as a jib slot problem. It actually had to do with (could be controlled by) adjustment of the main clew traveller and having stiff enough battens. It was actually caused by heavy leach pressure on the sail levering the middle of the sail to windward using as a fulcrum the line from the mainsheet/mainsail attachment point to the upper mast. The 3 main factors that contributed to this behavior were:

1. a boomless rig
2. a very roachy mainsail
3. a soft (very bendy) upper mast (above the hounds and diamonds)