Comment 18 for bug 1867092

The day before yesterday, I got the error as originally reported, except I was running Kubuntu 20.04, got the updates available prompt, opened Discover, updated shim and shim-signed, waited for both the regular software update and grub update to finish, and then restarted.

I posted a question on Ask Ubuntu, then found a CentOS bug with a fix I was able to adapt to my own situation and answer my own question. If you're a bug fixer, read the first note on the CentOS bug; if you've got this same problem, read the Ask Ubuntu question and answer:

https://bugs.centos.org/view.php?id=15522#c33203
https://askubuntu.com/questions/1277828/how-do-i-fix-broken-boot-after-shim-update