Jira Cloud Connect App does not show inside of Jira and throws 404 Error


I was just trying to go through the following tutorial, but it throws an error when opening the app inside of jira:

I configured the atlassian-connect.json, added the correct route of the ngrok tunnel to it and set the url to /hello-world
The app is available on https:///hello-world and shows the proper content and also the descriptor file under https:///atlassian-connect.json
The app also installs correctly, but if I want to open the App in Jira, it is loading for about 1 minute and throws then the following errors: (and it also says, something went wrong and I should reach out to the support)

Does anyone know how to solve this error? I also tried to reinstall the app after changing the atlassian-connect.json-file…