Hi Alexander and thank you for your post.

In order to set HTTPS without the GUI, you could do the configuration in an alternative way:
On the command line please issue the following commands:

sudo openproject config:set OPENPROJECT_HTTPS=true
sudo openproject configure

This should resolve the yellow banner to disappear on the bottom of the app in the browser window.
I hope it helps.
Good luck.

Hi Alexander, did it work now?

Hi Alexander, 

there is another solution that might work better,
if you start the wizard by using 

openproject reconfigure

and at the "server/ssl" screen you chose Yes

Be aware you need to supply the certificates there.

BR Adam

Hi Alexander

You could maybe try to update to OpenProject 12.2.1.
Here the release notes: https://www.openproject.org/docs/release-notes/12-2-1/

Could you please confirm if it solved your issue.
Good luck!