I have found a solution for this bug: users-admin does not completely delay the entries of the user from /etc/passwd and /etc/groups. When you delay manually these entries, you can add newly an prevously delayed user.
-- [users-admin] Adding a previously existing user fails silently https://launchpad.net/bugs/66051 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
