Thanks for pointing that out, I didn’t realize that there is no supoort in paid versions, if its below 20Mio LoC or additional 20k.
Anyways, we finaly have our paid instance of SonarQube up and running on a new server and having the very same issue.
I also checked out: SonarQube Users: Let's talk about IIS and SAML Authentication! and configured IIS as described in Operating the Server | SonarQube Docs that links to Configure SSL for SonarQube on Windows
Maybe @Colin might have an idea?