I kept reading and working on this and the situation has changed. I followed the instructions here: Error: file '/boot/vmlinuz-5.12-X86-64' not found you need to load the kernel first - #6 by freggel.doe
What I did was this:
sudo su -
manjaro-chroot -a
then
rm -f /var/lib/pacman/db.lck
pacman -Syu
With no result.
Then I did this:
$ sudo mkinitcpio -P
$ sudo update-grub
and after rebooting my system goes into Setup mode. I do not get the original message, which I think is acutally somehow worse. At least before I could see my grub menu, but now I don’t see anything but the Setup screen. When I navigate to uefi-grub and select it, nothing happens.