Syntax of univention-run-join-scripts -dcpwd

On a BACKUP DC I try to run univention-run-join-scipts with -dcpwd parameter like this:

/usr/sbin/univention-run-join-scripts -dcaccount Administrator -dcpwd /root/pwfile --force --run-scripts 92univention-management-console-web-server

The file /root/pwfile just contains the mandatory password. Anyway, I end up like this:

Search LDAP binddn: 

* Running join scripts failed!                                           *
* Message:  Invalid credentials

Is there a specific syntax, or access rights for the pwfile? Omitting -dcpwd the join-script finishes up successfully.