Narrowing it down
The problem comes from the fact that I added 2 "charges" to the e-cart object last week.
These charges work perfectly, but they are what is causing this error. I did what you suggested and deleted checkout.php, checkout_success.php, checkout_failure.php, confirm.php, shipping_failure.php, pp_confirm.php, pp_cancel.php, pp_checkout_success.php, and pp_checkout_failure.php, after backing up the colies I had.
I then regenerated them using the wizard, copied all the settings from the other version I had already done, and posted the new pages. I got the same failed transaction. I tried a number of different settings, and still nothing worked.
the ONLY difference was that I was NOT able to get ANY PayPal error message code to work on the failure page. Anyway... the last thing I tried was removing the CHARGES from the e-cart object and reloading it using the newly generated pages. That worked fine. I then reloaded the OLD pages using the new e-cart object with the charges removed. THAT worked fine.
I have now reloaded back my old (very customized) pages again, since I know that they werent the problem.
NOW I need help.