Can't login with PAM plugin

I uninstalled all zarafa packages and installed the debian 7.0 kopano server packages. After modifying the configuration files I cannot login anymore via webapp.

server.cfg:

user_plugin = unix
auth_method = pam
pam_service = passwd

The errors:
/var/log/kopano/server.log:
Fri Jun 9 17:11:19 2017: [warning] Authentication through PAM failed for user “tester”: Authentication failure
Fri Jun 9 17:11:19 2017: [warning] Failed to authenticate user “tester” from “file:///var/run/kopano/server.sock” using program “apache2”

/var/log/auth.log:
Jun 9 17:11:16 srv2 unix_chkpwd[6136]: check pass; user unknown
Jun 9 17:11:16 srv2 unix_chkpwd[6136]: password check failed for user (tester)
Jun 9 17:11:16 srv2 kopano-server: pam_unix(passwd:auth): authentication failure; logname= uid=998 euid=998 tty= ruser= rhost= user=tester

The user is known to kopano and the entered password is correct:
~# kopano-admin -l |grep test
tester tester

Additional information, maybe related:
/var/log/kopano/server.log
Fri Jun 9 17:10:48 2017: [warning] getspname_r: Permission denied
Fri Jun 9 17:11:19 2017: [warning] Previous message logged 33 times

Please help urgently! Thanks!

Replying to myself. When running kopano-server as user “root” then PAM authentication works as expected. Is this a bug or a configuration error?