Hi folks, rethinking and some checks showed me the solution.
In /etc I found a file "fstab.boot.readonly" with the same content as fstab. I suppose, this file was also read during boot and as there was an entry for efi missing, it might create one. Not sure about it! Moving any fstab.* to another place (except of /etc/fstab), did a grub-update for safety and rebooted, everything is now as it should be. So the problem is soved! I am not sure, where this "fstab.boot.readonly" appeared, but I guess, it was created by the debian-installer-cd in rescuemode. Anyway, thank you all for your help and responses, they helped much, too. This problem is now solved! Best regards Hans
