In our experience it happens from time to time that one gets a "white screen of death" - short WSOD - after the login with the admin account. Most frequently this happens after the move of a Drupal website to a new host or an upgrade to a new release. This is of course very odd and can lead to a very time consuming troubleshooting. Unfortunately we did not find out the precise causes and we couldn't find clear answers either.
First, add the following to settings.php what eventually might give you more information about the error: $config['system.logging']['error_level']='verbose';
Processing the following set of measures almost always helps us to resolve the problem.
- Run update.php (After login and white screen go to www.domain.xxx/update.php and follow the instructions)
- Run cron and clear caches (1-9) with drush
- Deletion of one of the duplicated ctools folder in sites/all/modules or sites/all/modules/contrib
- Incrementation of PHP Memory limit to at least 256M
- Check-up PHP version. Upgrade to recommended version for Drupal release. Sometimes also a downgrade can help, for example from PHP 7.2 to PHP 7.1 for Drupal 8.5.x or from PHP 7.2/7.1 to 7.0 for Drupal 7.6x
- Check if a PHP.ini file has been removed by the upgrade
- Copying of original .htaccess and settings.php from Drupal installation
- Check-up and correction of user rights, 755 and 440 special files
- Make sure there is no module conflict, eventually first actualize all modules before the upgrade/move
Furthermore there is a workaround for the login: Create an editor with almost full rights - you have to uncheck "flush caches" and "Use the administration pages and help" - before the move, allowing you to login and perform almost all the operations.