Here is an update. After booting again using UEFI, I ran grub-install.
When rebooting to the new system, the passphrase is asked. Then:
ERROR: device /dev/mapper/vg--root-root not found.
mount: /new_root: no filesystem type specified.
You are now being dropped into an emergency shell.
rootfs #
Maybe I forgot a lvm hook for mkinitcpio? Now checking for that…
EDIT: I think the basestrap command was missing the lvm2 package. And the HOOKS was also missing lvm2. Now trying to fix it… 