Yesterday, I had an issue with my laptop: it didn't boot any longer. On startup, grub was started, but not with the normal menu, but just with a command line.
When I tried to enter commands (which weren't echoed), it gave me the error mentionned in the title.
I then booted the PC with a resue USB stick tried to re-install grub, but that didn't help either.
I give the solution in an answer below, but would like to know what was the real reason...