How to find the error that causes a white screen when registering a user?

Question

Drupal

On the site, when registering, the user enters a minimum of data, after which he presses the register button and a white screen appears. How to understand why it appears?
PS I tried to insert the error output code into index.php, but it doesn't show anything, there are no errors in the site logs (Recent log messages).

error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);

UPD
I found the reason - in the rules there was a rule that, after registering a new user, launched crons using the following code:

<?php drupal_cron_run();?>

Tell me how to fix the code so that there is no white screen? Is it possible to insert a delay in seconds?

Answer 1

View web server logs. There is definitely something more global, like "I can't execute index.php because there is no yy module" or something like that.

Answer 2

Drupal 7?
Try to enable error display in Drupal /admin/config/development/logging
Clear the cache and try to register the user again (for example, in incognito mode of the same browser).