@Kieran_Levin may have mentioned in passing what my problem is:
I do remember playing around with secure boot and I remember deleting EFI keys. I have a sinking feeling I may have wiped the DB key. 
That would explain the odd error message.
BUT I remember seeing the DB keys, not knowing what they were but observing that they looked long and complicated and thinking that I’d better not mess around with them.
You released this (thanks!) to check and recover the DB key. Looks complicated and I don’t know anything about this, but I’ll try to figure it out.