After some tinkering I was able to restore the access, I will leave the details in hope it could ever help anyone.
I found out that OC is not compatible with PHP8 as of now: OwnCloud 10.5.0.10 and PHP 8.*
I disabled php 8 and re-enabled PHP 7.3:
sudo a2dismod php8.0
sudo a2enmod php7.3
sudo service apache2 restart
// Set alternative name path.
sudo update-alternatives -- set php /usr/bin/php7 .3
sudo update-alternatives -- set phar /usr/bin/phar7 .3
sudo update-alternatives -- set phar.phar /usr/bin/phar .phar7.3
And after that the apcu module still was reported as not found, so I re-installed it:
sudo apt-get install php7.3-apcu -y
After this, I was able to login again to the OC login page.