The first thing that you might want to do is download the latest release of this version of eCart, 4.5.2. The checkout server behavior may have some updates to it.
If you continue to get this problem you will need to check the error messages and statuses for your checkout. You should be able to do this on your failure page by going to the bindings and dragging and dropping the error message and status bindings onto the page along with any other values that you think would help you.