I think the UEFI boot loader in
the PC is one of the corrupt one (will only boot MS Windows 10, not
Linux Mint 18, Debian 8.6 or Debian stretch).
I think I have found a bit more about the issue,
if I run sudo grub-install --force /dev/sda it works (the two errors are converted into warnings, but Grub is installed.
... but this might give errors when a new kernel is installed. since Grub propably not willbe upgraded, - Good there are an old kernel version too.
Since root is on an ecrypted device it is not so easy to make a static grub entry to the symlinks /vmlinuz and /initrd.img
Should I report this as an error, and where?