QPopper requires the library "/lib/security/pam_pwdb.so" if built with PAM authentication. The latest emerge of PAM seems to remove that library and results in QPopper failing. A temporary solution is to emerge QPopper with USE="-pam" and that will result in a QPopper without PAM support. It would be best for the future however to either have pam_pwdb.so added back to PAM or patch QPopper to use another PAM library like pam_unix.so.
Add 'pwdb' to your USE flags, this worked for me.
# grep pwdb /usr/portage/profiles/use.local.desc sys-libs/pam:pwdb - If you want pam_pwdb.so installed to use pwdb as passwd db resolve as invalid.
