The best way to determine if the issue is being caused by a theme and/or plugin is to do the following:
1. Temporarily switch your theme back to the Storefront (https://www.woocommerce.com/storefront/) or TwentyNineteen (https://www.ads-software.com/themes/twentynineteen/) Theme.
2. Disable all plugins except for WooCommerce.
3. Test to see if the issue has been resolved.
If that resolves the issue, then one-by-one you can switch back your theme and re-enable your plugins, testing in between, until you find the one causing the conflict.
If doing this on your live site is not possible, can you create a duplicate site? We suggest using WP Staging (https://www.ads-software.com/plugins/wp-staging/) if your hosting provider does not offer a staging server as an option.
I hope that helps you to figure it out.
Feel free to get back to us if you have further questions.
Further discussion on this thread – https://www.ads-software.com/support/topic/random-404-during-checkout-process/
Apologies for the multiple posting.
]]>