Matthias, I don't know what the error is, but have you updated your chart database?
Also if you are zooming in or out, new senc files get made for vector charts
. Next time you zoom in that area it will use thise senc files. You can tell opencpn to create the all the senc files in by hitting a button. To learn how go to the manual and look for Toolbar - Options - Charts
- Chart Tab and read. Also read about vector charts.