Thank you for your help! As it turns out, several mobo settings, settings which I have never had to modify before, had to be changed:
First, my X570 MSI motherboard has a CSM/UEFI setting which had to be changed to UEFI only. I rebooted, and the “shim_lock” error went away, but then the complaint was my “secure boot” settings.
So I changed the secure boot setting and tried again. This time, the “shim” error came back! Then I realized that when I changed the boot setting to UEFI only, I forgot to double check the boot order. Sure enough, “Manjaro” was second. I changed it to the first position and it booted normally!
I guess the takeaway here is that the recent update to GRUB mandated certain setting in the BIOS which were not required before.
And now my joy has returned. Thank you!