Comment 26 for bug 1821630

Tested this issue once more by enabling secure boot in the BIOS, and booting into Ubuntu 19.04 through GRUB2, no success as previously.

"Boot failure : a proper digital signature was not found.
One of the files on the selected boot device was rejected by the Secure Boot feature."

I believe that only Windows operating systems and their bootloaders have the right signature as they booted previously with secure boot enabled - before installing Ubuntu. (Some 9-10 months ago)

Does Ubuntu install these digital signatures required by Secure boot after its installation? I think Ubuntu probably did not install them, as it haven't found a way to do so, due to unrecognised/unusual "Secure Boot System". But I have no idea how all this Secure Boot works, these are just guesses.

Or Maybe the digital signature of Ubuntu is corrupted due to unsuccessful entry insertion: (If that even happened)

[key 1]
  [SHA-256]
  0000000000000000000000000000000000000000000000000000000000000000