Can't authorize service account on vCO

Hi,

I have a vCO instance, on which an AD AdminGroup is authorized as the vCO Admins:

This worked fine so far, but I don't want to schedule Workflow runs using a personal account. I am also in posession of a service account, which has the administrator role designated on the vCenter. This service account in not a member of the DOMAIN\AdminGroup. I thought the solution would be to create a new Group in SSO, like this:

So in the end in the vCO configuration I have changed the authorization to:

The test login is successful, but every time I want to log in via the vCO Client or run a workflow in the service account context, I get an error stating, that the account is not authorized.

What could be wrong in the config above? Is it not possible at all to authorize a single account but only groups?