“Users need to be modified in the Domain administration in order to use this service.”
Please have a look at the Users settings in the Univention Management Console. OX adds its own section to the “accounts” tab where you can enable a user and define specific rights for the appication.
Error message
The screenshot indicates that the AppSuite is not (yet) working. If the error persists I’d look at /var/log/open-xchange/open-xchange.log.0 for a hint.