Okay, I can reproduce the issue on an upgraded system, but only if I don’t restart any services after the upgrade.
So systemctl restart foreman apache2 fixed it for me.
Do I assume correctly, that the upgrade you did was just editing /etc/apt/sources.list.d/deb_theforeman_org.list and running apt update && apt upgrade? Not re-running the installer or anything after that?
There is nothing wrong with that approach right now, although we want to deprecate the support for it in the future.