Dear Community,
we are trying to set an application link between our confluence server and our jira cloud. It worked before with http. Since we changed to https on our confluence server using a bought certificate using reverse proxy we get the error seen in the screenshot:

The reverse proxy is configured according to the sources given in the documentation such as https://confluence.atlassian.com/kb/reverse-proxy-and-application-link-troubleshooting-guide-719095279.html
Afaik we have no access to the jira cloud logs and the confluence logs show nothing. We don't wont to bypass the https connection.
We are using Confluence Server v7.0.3.
Edit: we tried both OAuth on JIRA Cloud.
Edit2: the Cert in question can be found here: https://doc.tim-solutions.de/
Thank you for your time and effort in advance.
Cheers Philipp