Unable to add system admins from the Active Directory

Hello,

in order to access an on-premise SSAS datatbase via the Power BI Enterprise Gateway I have to add a user from an AC as a system administrator.

I can find the appropriate domain user in the AD with the object picker and add it to the system admins. When I press OK I get an error message, saying that the domain account does not exists, See full error message below (in German).

I can't explain the user should be unknown when the object picker can find it without issue.

Error message:

"Die Administrators-Rolle enthält ein Domänenkonto, das nicht vorhanden ist. Das folgende Domänenbenutzerkonto ist für folgende Rolle nicht mehr gültig: 'CUBICA\dev'.
 (Microsoft.AnalysisServices)"