Hi!
That said, have you made sure to mount the EFI ESP partition as /boot/efi?
I've just create an partition in the installer with the type `EFI`. Is this what you mean, or do you mean something else?
There are quite a few mentions of this error spread over all kinds of distributions (just try asking your search engine of choice for debian "grubd-install dummy"). E.g.: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=865417
Thanks for the tip! I'll read trough them and see if there's a solution. I'll also test to change some settings in BIOS.
Regards, Albin