Continue button on checkout.php not working

I'm currently building my first ecommerce store from scratch following the instructions in your extremely useful eCart webinar. I've used the Checkout Wizard and am currently testing everything. When I get to the checkout.php page, I have two problems - if I select the card payment options, the Continue button shows the loader but then seems to redirect back to itself; if I select Paypal I get the checkout failure page. I used a line of code found elsewhere in this forum to try to identify what the error is and the failure page reports all the correct info submitted but then at the bottom of the page it says:

["ECO_Auth_Result"]=>
string(2061) "2016-01-05T14:27:34ZFailure7fa7ac2e5ef15Security errorSecurity header is not valid10002Error63.018308778"
}

Is this likely to be because the SSL Certificate isn't set up yet (in progress at the moment), or maybe a problem with my Paypal Sandbox API credentials? But then that doesn't explain why the Continue button doesn't work at all when choosing the card payment option...

Any help appreciated!