when i upgraded from 1.7.0 to 1..3, i noticed that the authdaemon looks for /var/lib/courier/authdaemon/ directory, but the ebuild script seems to think the directory is needed in /var/lib/courier-imap/authdaemon/ either ebuild or authdaemon use the wrong directory (copying /var/lib/courier-imap/authdaemon to /var/lib/courier/ solved my problems)
My courier-imap-1.7.3 install uses /var/lib/courier-imap/authdaemon quite happily.