ref: https://wordpress.org/support/topic/fatal-error-on-last-version-4-0/#post-12531873

Table alexandr121131_wp_2h3tp.TOTscth0_actionscheduler_actions doesn’t exist in /var/www/vhosts/headplus.gr/httpdocs/wp-content/plugins/woocommerce/packages/action-scheduler/classes/migration/ActionScheduler_DBStoreMigrator.php:44
Stack trace:

1. /var/www/vhosts/headplus.gr/httpdocs/wp-content/plugins/woocommerce/packages/action-scheduler/classes/data-stores/ActionScheduler_HybridStore.php(225): ActionScheduler_DBStoreMigrator->save_action(Object(ActionScheduler_Action), NULL)
2. /var/www/vhosts/headplus.gr/httpdocs/wp-content/plugins/woocommerce/packages/action-scheduler/classes/ActionScheduler_ActionFactory.php(177): ActionScheduler_HybridStore->save_action(Object(ActionScheduler_Action))
3. /var/www/vhosts/headplus.gr/httpdocs/wp-content/plugins/woocommerce/packages/action-scheduler/classes/ActionScheduler_ActionFactory.php(84): ActionScheduler_A in /var/www/vhosts/headplus.gr/httpdocs/wp-content/plugins/woocommerce/packages/action-scheduler/classes/migration/ActionScheduler_DBStoreMigrator.php on line 44

after that i rolled back on 3.9.3 version and everything works…

Another customer reported seeing this on WP Engine, so maybe we'd be able to reproduce there?