The option --generate-sea=no-sea-sectors,extend-sea-sectors,close-gaps=1000 produces an error during map calculation here.

The option --generate-sea=multipolygon,extend-sea-sectors as well as --generate-sea=multipolygon,extend-sea-sectors,close-gaps=1000 produce no errors. Both produce less sea-artifacts as in the beginning. But in all versions , a big tile in the north east corner shows completely water where is is mainly land (south sweden).

What do you mean with this? Here the right (east) boundary is intersecting the baltic sea and the upper (north) boundary lies within sweden, norway and the north sea. So I would say, that the coastline hits the boundaries. Can you explain this in more detail?