Story closed: Finally, I made a backup of all data I could grab and set-up a new OMV.
My ratio: Updates, upgrades from within OMV-Extras and a new FW driver for the raid system (Micron) finally created three times different /dev/sda1 entries in /etc/fstab.
Editing fstab resulted in files showing-up again, but access rights could not be administrated anymore.
So long story short: Get all data you could grab, set-up a new server and there you go again.
This time I was in luck: No data loss, just one weekend. 8:[