I have since raising this issue - have had the issue resolved. I have made no changes to my website - both ZenCart and WooCommerce platforms at all. The only changes I made - which I am positive would not have made a difference, is re-initialise the API's signatures etc for PayPal. The old ones were running since 2010, and without any issues at all. However PayPal suggested that I try and re-initialise and this may resolve. Upon re-initialising and making the changes to WooCommerce and ZenCart - PayPal - modules - it continued to do the same. Some hours later - it was all resolved, without me making any changes. I had placed a call with the PayPal MTS team, and they tested it for me, and reported back that they could not see the issue at all. Since then it has been working.
... View more