Magento's 'Paypal Website Payments Pro Hosted Solution' Bug Fix

If you want a testimonial saying that you are THE best Magento Devs on UpWork, then take this challenge! For some reason about 20% of attempted transactions fail. Customers are redirected to a 404 on the client site, when they either attempt to pay by logging into paypal and processing the payment; or by filling in the Paypal credit card detail info and submitting to Paypal. Apparently there are no problems with any files, yet some transactions still fail. Paypal's log shows failed payments as 'success', yet no money lands in Paypal and the customer is returned to a 404 page. API credentials were re-input and still the problem persists. Front end example of a failed transaction can be seen here: [login to view URL] I need someone to investigate and resolve the problem and you will be required to provide a full log of all changes made to resolve the problem. This is a re-posting because the previous 2 developers hired here couldn't fix the problem! Thank you and look forward to hearing from you Magento/Paypal Pro Hosted Solution integration heroes!