Just to finish off this thread (for anyone else that is stuck with this issue!) here is what we did to fix it.. (you will need webassist ecart)..
Fix by Jimmy Wu:
We were able to take a look at this issue over a connect meeting session and resolve the issue. The steps we took were to install eCart 4 and reapply the server behaviors for paypal express checkout on the checkout.php and pp_confirm.php pages and hardcode the user, password and signature.
On the checkout.php page, the server behavior to reapply is the Authentication for PayPal Express Checkout server behavior. You should click on the UserName Field Name, and change the value to the hardcoded value. Hit tab after entering in the information and make sure the value is changed in the display. Repeat this process for the Password and Signature/Vendor Field Names as well.
On the pp_confirm.php page, the server behaviors to reapply are the Process Transaction with PayPal Express Checkout and Get Payer Profile from PayPal Express Checkout server behaviors. Change the Username, Password and Signature/Vendor fields in both and nothing else. To make sure the values stick, hit tab after entering in each value.