ejabberd and pam debugging?

[christian-ETYuwdzrPJK4mfPA/iJWtA at public.gmane.org (Chris Ampenberger)]

I installed ejabberd 2.1.5 as an internal xmpp server on a gentoo system
and have problems to get it running with pam authentication:

In /etc/jabber/ejabberd.cfg:

    {auth_method, pam}.
    {pam_service, "xmpp"}.

The corresponding pam service file is /etc/pam.d/xmpp, which is auto
generated and contains:

    auth           include         system-auth
    account        include         system-auth

The included system-auth is

    auth        required    pam_env.so
    auth        required    pam_unix.so try_first_pass likeauth nullok
=20
    account        required    pam_unix.so
=20
    password    required    pam_cracklib.so difok=3D2 minlen=3D8 dcredit=3D=
2
ocredit=3D2 retry=3D3
    password    required    pam_unix.so try_first_pass use_authtok
nullok sha512 shadow
    session        required    pam_limits.so
    session        required    pam_env.so
    session        required    pam_unix.so
    session        optional    pam_permit.so

When I use pidgin to login with a valid user and password, I get the
following in syslog:

    Aug 26 22:34:35 sokrates unix_chkpwd[3351]: check pass; user unknown
    Aug 26 22:34:35 sokrates unix_chkpwd[3351]: password check failed
for user (whoever)
    Aug 26 22:34:35 sokrates epam: pam_unix(xmpp:auth): authentication
failure; logname=3D uid=3D109 euid=3D109 tty=3D ruser=3Dwhoever rhost=3D =
 user=3Dwhoever

I know user name and password are correct. Any suggestions, how to
troubleshoot that best?