How to access the App Store?

I have my nextcloud instance up and running, but it seems I can’t access the appstore. When I click on Apps as admin, I get a window with 6 categories:

Your Apps
Active Apps
Disabled Apps
App bundles
Featured Apps
Developer documentation

That’s it. I recall that there were more categories once and that I could somehow browse available apps. In fact, I have the “Deck” app installed; thus the app store did work at some point in the past.

How can I troubleshoot this?