Maybe this message helps a bit?
==> /var/log/apache2/mail.<xxx>.net.error.log <==
[Mon Feb 25 11:03:34 2013] [error] [client 85.183.15.4] user postmaster@<xxx>.<xxx> not found: /cluebringer/
The LDAP scheme was imported from an older iRedMail installation.
But all administrative Accounts have the correct password, I updated them via phpLdapAdmin.
Best,
Achim