SSL Error when trying to connect self hosted Gitlab with JiraCloud

We are currently trying to connect our instance of Jira Cloud with our self hosted instance of Gitlab. To achieve this we hosted our own app-descriptor file on our own server. Following the steps described in this guide: for Jira Cloud app | GitLab, we put our Jira in dev mode and tried installing the app from our app-decriptor.json. Here Jira either shows a unexpected error and the webpage freezes, or it states that an SSL error occured. When we try to access the link to the app-descriptor file directly it does show us that the CA Certificate is valid and that a secure connection was established.
Any idea what is causing this error?
I checked the links within the app descriptor file and none of them seemed out of the ordinary.

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Does no one have an Idea what could be causing this? If more information is required, please ask for it. I will gladly answer.