I think we should doubt cache since Drupal 8 aggressively uses cache-backend mechanism. If you encounter a fatal error or WSOD (White Screen of Death), check the following items.
Check error logs at /var/log/apache2/error.log
sudo tail -f /var/log/apache2/error.log
Configuration | Performance - Check off Aggregate CSS files and Aggregate JavaScript files
Configuration | Performance - Clear all caches
Rebuild cache from Drush
# Move to the directory where Drupal 8 is installed by using cd command,
sudo drush -l <YOUR_SITE_URL> cr
Try each cache from Drush, too.
# Move to the directory where Drupal 8 is installed by using cd command,
sudo drush -l <YOUR_SITE_URL> cc
Rebuild cache from Drupal console
# Move to the directory where Drupal 8 is installed by using cd command,
sudo drupal -l=<YOUR_SITE_URL> cr
IMPORTANT: You may want to clear cache-related tables on your database:
TRUNCATE `cachetags`;
TRUNCATE `cache_bootstrap`;
TRUNCATE `cache_config`;
TRUNCATE `cache_container`;
TRUNCATE `cache_data`;
TRUNCATE `cache_default`;
TRUNCATE `cache_discovery`;
TRUNCATE `cache_dynamic_page_cache`;
TRUNCATE `cache_entity`;
TRUNCATE `cache_library`;
TRUNCATE `cache_menu`;
TRUNCATE `cache_render`;
TRUNCATE `cache_static`;
TRUNCATE `cache_toolbar`;
You may delete the following files/directories (sites/default/files/css or js or php).
sudo rm -fr sites/default/files/css; sudo rm -fr sites/default/files/js; sudo rm -fr sites/default/files/php
Restart apache2 because the cache might exist in memory in case of Class Not Found Error. You may also want to restart memcached and mysql server.
sudo service apache2 restart; sudo service memcached restart; sudo service mysql restart
Check the remaining disk space.
sudo df -h
Finally, try to reboot your server if you try above all.
sudo reboot