My OpenCart store shows a blank white page, what causes it?

A blank white page (WSOD) almost always means a PHP fatal error is being hidden. The cause is in storage/logs/error.log, commonly Fatal error: Uncaught Error: Call to a member function ... on null, a missing class after a half-finished extension install, a memory-limit exhaustion, or a broken vQmod/OCMOD modification. We tail the log, trace the controller and model, and patch the real cause, we don’t just hide the error with display_errors off.