Hi @markus-karthaus_8928,
The validity check of a certificate ion ProGet is primarily to verify the certificate itself is valid, not if it is valid for ProGet. Any self-signed or internal domain certificate will be invalid by default unless the certificate or certificate authority exists in the trusted root on your server. If it is a purchased certificate, I would check that your certificate's chain is properly installed on your server. If your certificate is a valid certificate but requires a custom certificate chain (many do), that chain will need to be installed on the server for ProGet to validate that properly. A .pfx certificate does not store the certificate chain internally in the file. The browser handles the validation slightly differently, so that is most likely why it seems to work in the browser.
When it comes to the .pem file. There are many ways to generate it, but I'm guessing the certificate chain was stored internally in the pem file, which then does not require the certificate chain to be installed on the server.
I'm speculating on the certificate chain in these cases because seeing why your certificate is not valid requires more than the screenshots you provided. I would actually need to see your certificate itself to truly validate this.
Lastly, when it comes to using a .pem file, .NET tends to be very picky about it's format. It is not as forgiving as other frameworks. If you look in the "HTTPS Binding to a Port (Advanced) (Experimental)" of our HTTPS Support on Windows documentation, we have instructions on how to create a .pem file from a .pfx. I'm not sure if that is what you followed, but that is the simplest way we have found to generate a .pem file that works with .NET.
Hope this helps!
Thanks,
Rich