Is the account using PHP-FPM as it's PHP handler? If so, check to see if you notice any errors in the following log file:
I'm having a similiar issue. Moving a user from a system to a new system, and they're getting 500 errors that aren't logging. I've enabled display_errors, checked the local error_log, /usr/local/apache/logs/error_log, and in /home/USER/logs/
The user wants the old condition back where if there was a PHP error on their server it would display in the browser. Now we can't even get the error to log on the new server, but a 500 is replied.