I have also reset hsts for the subdomain in the browser using net-internal, unfortunately without success.
The certificate works fine on the main domain.
In the Cloudflare DNS I also have a wildcard entry which points to the IP of my server.
Thanks for your quick reply!
I guess that is then related to the fact that I do not have a specific DNS record for “staging” but a " * " record, which can not be proxied in the Free Plan.
I have now changed it as you described. a separate record for staging and this is proxied. Now everything works!