Comment 15 for bug 1848797

I too had this problem. Same error message in the grub prompt.

I had to burn a live-cd, and boot from it. Reapairing grub from live-system didn't work, it "failed to get canonical path of /cow". So I reinstalled the whole System while keeping all my settings, files, stuff in /home, and so on

that worked quite well and getting things back to normal is done in no time.

I know, this is no solution to the bug, but I think it's necessary to give people a hint how to at least access their systems again.