Hi j-ed,
Thankyou for pointing me in correct direction. I was looking at wrong logs for debugging the issue.
To point out what actually happened and what the solution was, please read the following:
When updating ubuntu and nextcloud, some how my php modules in /etc/php7.2/mods-available were carried over to /etc/php7.3/mods-available/
This includes Redis and APCu modules as well.
However, Apache2 was still using php-7.2 (with missing packages) and not php-7.3.
The error in my nextcloud.log was “APCu not available for local cache” and hint was “Is the matching PHP module installed and enabled?”
sudo a2dismod php7.2
sudo a2enmod php7.3
sudo systemctl restart apache2
So to resolve this I disabled PHP-7.2 and enabled PHP-7.3 and voila! It is working now.