I did put the two music-directories in one songs-database by adding another music-directory in the web-interface. After a full scan I created the index on artist.
Unfortunately the browse-error was there again. So even an index doesn’t solve the problem, arghh…
I did have some difficulties to remove the second music-directory again. I removed it from the web-interface and then started a full scan again. After this was finished still approx 23K artist were shown, it should have been approx 10K.
I did issue the VACUUM command on the SQLite and then again a full scan. After that the correct number of songs was displayed.