In case someone else runs into this: I have an encrypted LVM partition that contains mount points for root, home, swap. Apparently cryptsetup cannot see through the LVM to find the rootfs, and hence the error.
I added the `initramfs` option in /etc/crypttab and updated initramfs for safety, and everything worked fine on reboot. I didn't try without the option, so I can't say if it's required or not. - Jacob Post
