"Test a role as" functionality not working on my datamart's semantic model

Good day,

I have connected a datamart within powerbi service to an Azure SQL source. The data is transformed into Facts and Dimensions and then RLS is applied to the star model. Getting feedback and testing with users I am able to see that the RLS is working. But now when I want to view what a user sees (from a dashboard perspective), I usually go to the security settings of the semantic and then row-level security group and select "Test role as". 

The error I get is as below:

I have tried different connection types, but I don't understand how exactly this needs to be configured to enable this functionality again. It was working on an older datamart with the same credentionals and this functionality works.

Thanks in advance for any advice or assitance on the matter.