Frequent issues and their causes.
WHITE SCREEN
Enable error display temporarily in settings.php, or read the error log.
Frequently a module conflict, a PHP version incompatibility, or memory exhaustion. Drupal uses more memory than most applications.
TRUSTED HOST WARNING
Set the trusted_host_patterns value in settings.php to match your domain. Without it, Drupal warns continuously.
PERMISSION WARNINGS ON STATUS REPORT
The files directory needs to be writable; the settings file should not be. Drupal's status report states exactly what it expects.
CACHE PROBLEMS
Drupal caches aggressively. After configuration changes, clear the cache. Many apparent bugs are stale cache.
The cache can be cleared from the interface, or with Drush if you have SSH access.
DATABASE UPDATES AFTER AN UPGRADE
Visit the update script path after any core or module update. Skipping it leaves the database out of step with the code.
MEMORY EXHAUSTION
Raise memory_limit in cPanel. Drupal with several modules commonly needs more than the default.
COMPOSER FAILURES
Usually memory. Run Composer with a raised limit.
