It sounds as though there may have been problems contacting the Viessmann endpoint to refresh the access token, the access token is relatively short-lived, whereas the refresh token has a lifetime of ~6 months IIRC. I have had this happen once or twice when Viessmann’s servers have been down. The authorise button will send you to a Viessmann login web page, if that doesn’t render it sounds like the problem is at their end. It renders for me, so perhaps they have fixed it.
