PayPal express checkout problems, not working

[bab]

Error: Security header is not valid. This is the response I get when I try click the "Test" button within the extension set-up.

Error Log:
2013-10-17 8:22:40 - App Warning:  AbanteCart core v.1.1.7 PayPal Express Checkout Error: Security header is not valid. Test mode = 1. in /home/user1/public_html/extensions/paypal_express_checkout/admin/controller/responses/extension/paypal_express_checkout.php on line 82
2013-10-17 8:23:31 - App Warning:  AbanteCart core v.1.1.7 PayPal Express Checkout Error: Security header is not valid. Test mode = 1. in /home/user1/public_html/extensions/paypal_express_checkout/admin/controller/responses/extension/paypal_express_checkout.php on line 82
2013-10-17 8:27:45 - App Warning:  AbanteCart core v.1.1.7 PayPal Express Checkout Error: Security header is not valid. Test mode = 1. in /home/user1/public_html/extensions/paypal_express_checkout/admin/controller/responses/extension/paypal_express_checkout.php on line 82
Please help! no clue on what to do.

[abantecart]

Are you testing connection in the test (sandbox) mode ? Try to set Sandbox to NO.

[bab]

Ok, I turned off test mode, but it's still the same. Any ideas? Thank you for responding

[maxter]

This is a PayPal error

Try to release and then regenerate your API credentials from within the Paypal account.

[Basara]

2013-10-17 8:27:45 - App Warning:  AbanteCart core v.1.1.7 PayPal Express Checkout Error: S

Your cart version is very outdated as well PayPal Express extension

[Anastasiia H.]

I think that Paypal isn't the best payment system and there are a lot of alternatives. I have read the article about it "PAYMENT GATEWAY COMPARISON: GUIDE TO ECOMMERCE PAYMENTS". Maybe it will be useful for somebody. (Link is in the signature)