UEFI MOK Management failure
I have observed through the last monthes failures when booting.
EFI failure, file not found ...
"Failed to start MokManager: Not Found
Something has gone seriously wrong: import_mok_state() failed: Not Found"
The problem seems to be known ( https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1845466 ) and seems to be solved since GRUB 2.04.
My secure boot is disabled in BIOS.
I think I see that issue when Kernel updates have been done. (need to observe)
Boot is possible via USB. After boot from USB, problem is solved so far.