PayPal Express / Ecart 6 Not Working

I have experimented with this one and need help. I have used the Ecart Wizard to create the checkout process. Now, when the user gets to the checkout page they are offered PayPal Express as a payment method. However, upon selecting the PayPal Radio Button, the Pay with PayPal container / Continue Button slides up the page, but when the Continue Button is clicked, the overlay appears with the progress bar, but then goes back to showing the form with the pay options back to paying by card. It appears that it will not redirect to the PayPal page to collect the information, but I am just guessing. Need help as I am on deadline.

On a related matter, in the checkout wizard when I choose the payment process as PayPal (standard), the checkbox for including PayPal Express in the checkout is greyed out and is not included in the checkout page when viewing. THis kind of implies that one cannot include PayPal Express if the default Payment Process is PayPal, is that correct? If it is not correct, then there is also a problem in the wizard as when I choose PayPal (standard) I cannot get PayPal Express in to the checkout.

Paypal Express is not availbale as part of the PayPal standard checkout process.

it can only be added as a stand alone, or as part of the Paypal Payments Pro checkout process.

I'll need to troubleshoot directly, see the private message section.

in eCart 6, when running the checkout wizard, on the checkout tab, select the option to rely on the gateway to provide the secure checkout process, then select Paypal express as the gateway.

No you don't need SSL when using Paypal Express.

The response from paypal says:
Security errorSecurity header is not valid

this means there is a problem with the API Credentials being used.

it may be that you have not accepted the billing agreement after creating the API Credentials.

It usually happens if the Live API Credential are being passed to the sandbox or vice versa.

it could also be just a bad set of credentials, you may want to try to regenerate them.

It can take a while for the old credential to be come disabled and the new ones active, during that time, you may get the token error, wait a couple of hours and test again.