– renaming the synoindex to synoindex.old was the final cure;
– after a major update from the Synology firmware 0590 to 0637, firefly did not work at all (maybe I messed around with it too much 😉 )
– finally, I reinstalled lately the old firefly/mt-daapd server svn-1696 using the instruction from jchris below. Now the DS-207+ runs on the 0637 firmware flawlessly.
Good luck to you all!