Known issue (and solution) in eCart 4 with Sage Pay/Protx
There was a bug discovered in eCart 4 for the Sage Pay (Protx) VSP Server payment gateway. If you use this payment gateway and have experienced a checkout failure on the verification page, you will want to download the fix. The fix is provided in all 4 server languages via a file in the Download Center that you can just copy/paste to any of your sites that use this payment gateway.
Log in and visit the Download Center. Click on eCart 4; then click on the Known Issues section. Download the related zip file and follow the instructions in the readme file.