Orders not completing

Hello,
Hoping for some help or advice – as I feel like I have tried everything!!!

My website has stopped processing orders and PayPal payments. Payment is successfully taken by PayPal, but PayPal does not communicate back to my website to ‘complete’ the order. Orders remain as ‘pending payment’, so I am not notified of new orders until a PayPal notification comes through.

There are no order notes appearing on these orders either, so nothing is being taken out of stock.

I can also see in PayPal that the IPN is getting a 403 error trying to communicate back to my site.

I am also seeing thousands of these two lines in error log:

[27-Oct-2017 01:10:35 UTC] The WC_Cart::get_cart_url function is deprecated since version 2.5. Replace with wc_get_cart_url.
[27-Oct-2017 01:11:59 UTC] The WC_Cart::get_cart_url function is deprecated since version 2.5. Replace with wc_get_cart_url.
[27-Oct-2017 01:12:00 UTC] The WC_Cart::get_cart_url function is deprecated since version 2.5. Replace with wc_get_cart_url.

I have tried the following:
– Disabled all plugins, switched to twenty seventeen & woocommerce only, tested again and orders still not working
– Checked, double checked, turned off, disabled and re-enabled IPN on PayPal – orders still not working under any of these settings
– Turned off firewall settings on server, re-tested, still not working.
– Checked my WooCommerce system status – cURL version is correct and open, there are no WC page errors

All of the above errors (orders not completing, no order notes, PayPay IPN 403 error and the same error log message) still continue no matter what testing I try.

Am using WP 4.8.2 and WC 3.2.1

Is there anything else I can possibly do?
Appreciate any advice/support I can get – thanks in advance!!