domain mapping does not show "select as primary domain"

I created a new site on my network and added (mapped) four domains to it. All are shown as "valid". I want to define one of them as the "primary domain". There should be a star icon next to the "delete" icon that allowed me to "select as primary domain" one of them - but the star icon (and thus the option) does not show.

Do I miss anything?

Thanks for support!
Wolfgang