Today, after upgrading a client's WooCommerce to version 4.0.1, a database error occurred, showing issues like >save_action(Object(ActionScheduler_Action), NULL) #1. Based on feedback from other users on the official forum, this problem occurs because you have too many products (usually over 1,000).
The solution is to temporarily downgrade to WooCommerce 3.9.3 and wait for the next WooCommerce update to test if this bug is resolved.
Downgrade Method:
1. Download the old version ofWooCommerce 3.9.3 installation package
2. Overwrite the files of the existing WooCommerce plugin folder
3. Restore the backed-up database
References:
https://wordpress.org/support/topic/can-not-update-my-database/