So your Moodle site's URL is https://undervisning.net-akademiet.dk and this works as expected.
But if you try to access it at http://undervisning.net-akademiet.dk (HTTP not HTTPS) you get redirected to the non-existent URL.
I would guess that this is something Apache is configured to do. If you manage the server try to find the Apache .conf file that's listening on port 80 for this domain name, for example:
and look for a redirect rule, normally RewriteCond and RewriteRule.
If you manage the site via a web interface hopefully you can find the configuration setting for the non-HTTPS site.
I don't think this is something in Moodle that changed during the upgrade. Let's Encrypt has a script to install certificates which optionally adds a redirect so it's possibly something this has done.