I’ve been having this problem too.
In my case, I was seeing errors with WordPress SEO plugin:
Catchable fatal error: Argument 1 passed to WPSEO_OnPage::add_weekly_schedule() must be of the type array, null given in /wp-content/plugins/wordpress-seo/admin/onpage/class-onpage.php on line 105
It appears to be a result of /wp-spell-check/wpspellcheck.php, function cron_add_custom(), which runs on the filter cron_schedules. Approx line 2308-2350.
In some cases, this function has no return value, when the filter expects an array returned. This would happen when if (sizeof($check_db) == 0).
Finishing off the function with return $schedules; would probably fix the issue.
