Problems connecting to SSAS through On Premises Data Gateway

Hi,

I'm having problems connecting to SSAS from the app.powerbi.com via On Premises Gateway

Scenario:

• SSAS tablar is installed & On premises gateway are installed on the same machine. They will be on different machines later but we want to assure that there are no closed port issues between both servers for testing.
• The gateway works for passing through SQL Server queries.
• The SSAS database can be connectod to via the gateway management on app.powerbi.com.
• The SSAS database can be loaded into Power BI Desktop (and published - but then it breaks).
• The Data source can be found on app.powerbi.com -> create report -> get data -> sql server analysis services; but when we try to connect 'something went wrong'.

Error Message:

What could be the issue?