I had the same problem with an unstable/experimental system. The problem was, that no /etc/cups/passwd.md5 existed. This error also appeared in the error log. I could solve this problem through typing "lppasswd -a". After this everything worked. I think this should be done in some automatic way.
I hope i could help Andres Freund
pgpM5AvAbuEWd.pgp
Description: PGP signature