patti

Thank you for the hints and showing in the right direction. As you suggested I created a separate /boot partition for Debian and installed the system as before. Now I have one more (/boot) partition and after the install a nice Grub menu with all the entries (also Windows) greeted me. I figured I would have made it troublesome for the future without any crucial benefits if I put the debian /boot files on the ESP partition. Anyone wondering, here is how I did it: 1. First created a full hdd backup...

Thank you for the quick response. I suspect that's exactly the issue here. However it's not that there is encrypted boot, but there is no boot partition for Debian at all. It was my dilemma during the install: If I choose sda1 as boot partition for the linux system and reformat it, I'll lose the Windows EFI configuration on that partition. This is why I did not touch sda1, but I was trying to find a way to install the necessary files on that unencrypted partition afterwards (from the live system...