After Protx was taken over by SagePay there have been many changes taking place in the overall look and working of the company. Once of these changes has been a change in the URLs of SagePay Form payment method.
SagePay had informed all merchants and affiliates of this change 6 months back on their website as well as by email. They had requested to change the URLs to the new SagePay ones as the older ones would be taken down in the near future. However, to support a smooth change over they have provided backward support of old Protx URLs.
They have now informed all merchants to make the change by September 30 2010 or the old Protx URLs will not work any more and their website would not be able to charge customers.
They have also made available a page on their website detailing the old URLs as well as the new URLs to replace the old ones.
You can find the new SagePay URls on the SagePay website and follow them to make the changes to your website.
Hope the above helped but if you need my help in this regard then do let me know.