Thanks mornage! This worked! I emptied the "Album Artist" field and it is now looking as it should (so with only one artist name).

The Genres are still displayed double (e.g. Jazz, Jazz). I have no idea how to correct that. Anyway, I hardly use the Sort-by-Genre option so it is not a major issue for me.

It also appears that the songs of WMA albums are listed alphabetically, i.e. it ignores the track-number tag. After some searching I found out that this is in fact a well-known problem on the N900 (see earlier discussion) and is registered as bug 5900: see here, and only remedied by renaming the song-titles (i.e. to let them start with their track-number).

Thanks anyway a lot!