Security header is not valid using express checkout
I have an issue with eCart and Express checkout.
Setting up eCart with PayPal Standard using Express Checkout.
Set up ecart as PayPal Pro using Express Checkout credentials.
Once created, removed Credit Card details from checkout.php
Added Local Checkout Response for both Process and Authorise
When I choose Paypal express checkout I get Security header is not valid.
I have already changed the PayPal Express checkout API Credentials in my PayPal account and get the same output when submitting order.
If anyone could help with this i would be grateful