The solution is from this thread. I’ll post it here again, to make it easy, if someone else is looking for it here. It works very well for me and I hope it will help you, too.
Remove the dash separators for all cryptomount -u entries
cryptomount -u 3722dfb2-3b32-414b-bd59-4329fa92b6a9 (example)
to
cryptomount -u 3722dfb23b32414bbd594329fa92b6a9
in /boot/grub/grub.cfg
To edit the grub.cfg I had to
sudo chmod 644 /boot/grub/grub.cfg
Even on a fresh Manjaro installation with encryption that error appears after the first update and reboot. I hope they will fix that soon.