It sounds like your site is still trying to load the page through SSL/HTTPS, but your server is not setup for that.
When you choose PayPal Express, it shouldn't load the page over HTTPS.
Do you have another plugin that may force SSL/HTTPS? If so, disable or alter them.
Do any of your pages still have the "besecure" custom field? If so, remove that custom field.
Try the above. If you still have issues, you can absolutely tell PMPro to always use HTTP by adding something like:
$besecure = false;
However, if something else on your site is loading HTTPS, you will still have problems.