I would suggest that you make use of the latest release of eCart 4 to recreate the checkout server behaviors on these pages. I know that there were several changes made in the last releases of eCart 4 so this issue might resolve itself with the updated files.
This is still version 4 of eCart so updating should not cause any problems. Just make sure that you backup the site in it's current state with all of the files before you make any updates to it.
If you continue to experience any issues with the checkout it would be best to have some steps to consistently reproduce the issue, if the problem is with the code that is used to process the transaction is should be reproducible with a given set of steps. If the problem is with the values of a particular item or the values that the user is entering it might take some more effort to debug the issue. If you are getting sent to the checkout failure page then there should be some type of response that you should see, do you see the full request when you get to this page?