Power BI Desktop (RS) with Custom Security?

Hello Power BI Community!

after successfully applying the changes that needs to be done for the Custom Security sample from github (https://github.com/Microsoft/Reporting-Services/tree/master/CustomSecuritySample) I've created new accounts in the database and everythins works fine when logging into the webpage http://<server>/reports.

Power BI reports can be executed without problems but from Power BI Desktop (RS) there is no chance to open or save reports. The Error Message is in the connection dialog to the Server: "Unexpected error encountered"

Before applying the changes for Custom Security everything works fine.

How can I fix this?

Thanks and best Regards

Volker