Welcome screen loop and no functionality after fresh installation

I used Installatron to do a fresh install of Nextcloud 20 at my provider’s Apache server. I get the ‘Dashboard’ screen but then, whatever I try to do, I just get the Nextcloud Welcome screens (which I keep quitting by clicking ‘start using Nextcloud’). Basic functions do not work, e.g. setting status or location, loading files, contacts, or calendar entrees.
I am running PHP 7.4.
What could go wrong with a fresh install like this?