403 error when customers finish Paypal payment on my Woocommerce site

This error occurs after the customer checks out, they choose Paypal and are sent to a Paypal website to complete payment. After payment is completed they’re redirected again and it’s supposed to send them to the Order Completed page on my website. But instead they get the following 403 error message:

———————————-
Error 403

We’re sorry, but we could not fulfill your request for /checkout/order-
received/974?key=wc_order_5a00e4bbb47ff&utm_nooverride=1 on this server.

You do not have permission to access this server. Data may not be posted from offsite forms.

Your technical support key is: [a 16-digit key]

You can use this key to fix this problem yourself.

If you are unable to fix the problem yourself, please contact [my email address] and be sure to provide the technical support key shown above.
——————————————–

So since this happens after payment is made, the order still goes through successfully. I’m not losing sales to this bug, but it doesn’t look great to the customers.

I’ve tried adding URLs to the URL white list (I added “checkout/order-received/” and “paypal.com”), but I clearly don’t know how to use the white list correctly (and couldn’t find instructions).

Any help is appreciated!