Whenever I open the "Software" app, items appear normally but clicking on an application to see its details only show the loading icon. It'll keep loading forever. This happens to all the software items displayed.
And even though I have set the Software app to look for app updates daily, it now shows "Last checked: Two days ago". There is no option to search for them manually either. I do receive Zorin OS base updates from the "Software Updater" app, however.
I disabled ProtonVPN and its Netshield, but that doesn't help.
ProtonVPN is the culprit indeed. Uninstalling ProtonVPN and rebooting the system resolved the issue (naturally). I'll try to install it again and turn on the functions like the Kill Switch to see what exactly caused the issue.