Hi @Thor
Are you by any chance using the PHP repos from Sury? I had a similar problem this morning. There were updates for PHP7.4 packages available. After I upgraded them I also did the Upgrade of my Nextcloud to 21.0.3. Somehow the update of the PHP packages must have changed the PHP settings. I took the radical route and uninstalled PHP with “apt purge php*” and then reinstalled and configured it again.
However, I would first check whether the PHP-apcu module is still installed and active and the following lines are still present in… /etc/php/7.4/mods-available/apcu.ini
extension=apcu.so
apc.enable_cli=1
Hope that helps.