I have just downloaded the latest dnf upgrade that was offered to me this morning :
4.18.0-553.8.1.el8_10.x86_64
and now 553 works.
The mount boot/efi fail has disappeared
Within a very long list of updates, I noticed that it installed linux-firmware-20240111-121.gitb that I had never seen before.
Maybe it was this that was broken.
All very strange.
Thank you for all your help.