Unable to add Nextcloud account or sync Nextcloud addresses

I have a fresh install of Fedora 33 Plasma spin. When I try to add an online account from System settings → Online accounts, I only get an empty window.

And when I try to sync my contacts to my Nextcloud account from KOrganizer, I get a HTTP 0 error when trying to fetch the addressbooks. I can sync them to Evolution on the same install, but I’d rather use KDE apps.