Error Phpmyadmin
I found a fix (also Ubuntu 18.04), I downgraded the package php-symfony-polyfill-apcu back to 1.6.0-2. It appears the version that was upgraded no longer includes all the same files, including the 'Symfony/Polyfill/Apcu/autoload.php' file that is causing the issue here.
I had to reinstall phpmyadmin to fix the problem. Same Ubuntu server 18.04.
sudo apt remove phpmyadmin
sudo add-apt-repository ppa:phpmyadmin/ppa
sudo apt install phpmyadmin