This post will be short but hopefully be of help to you anyway!

Today I made some changes to my GRUB configuration. But when I ran update-grub, I found something like this in the output:

grub-probe: error: unknown filesystem.
/usr/bin/grub-probe: error: unknown filesystem.

The interweb is filled with explanations of what the cause might be. For me the reason simply was that I had - like always - an LVM snapshot in effect. Removing the snapshot resolved the issue!