Paypal is on an https:// protocol with SSL certificate for security. But your site use the http:// unsecure protocol.
So http:// to https:// with an iframe as done into the website viewer plugin can't be reached due to security rules of browsers. It's to avoid a hacker injection of the checkout url which could be redirect to a hacker site rather than to the paypal site.
So if your site use HTTPS protocol (need SSL private or shared on your hosting plan) you can use Paypal HTTP url also.
Thank you Benjamin for your answer. Please confirm, if I open a panatour within a https environment, a MEC Paypal link within the webbrower box within panatour will work? it is important to open a Mobile Express Checkout link (https)
Thank you again for your assistance, Friendly regards, Gerben