The domain is working perfectly fine for me. Things like generating an SSL certificate can always take some time, since the DNS settings need to clear.
If you’re still having issues on your end, I would recommend clearing your browser cache and potentially just well…waiting a bit. DNS isn’t a uniform thing that updates the same time for everybody. It might just be that your internet service provider is a bit slow to update the records or there is a DNS cache active somewhere.
Since you’re a paying Ghost(Pro) customer, I’d say, reach out to support@ghost.org, if you still see this message after 24 hours.