PopUp won't recognise Membership 2 Pro

I can create popup's for site visitors who are not logged in, but I'm getting errors when trying to create a 2nd membership level popup.

This appears at the top of the page:
cURL error 56: Failure when receiving data from the peer

And I've attached a screen grab to show the red boxed messages too, suggesting there's an issue with popup communicating with membership. (I have M2Pro)

Thank you!