Unable to preview popup using PopUp Pro

Hi,
I bought, downloaded & installed PopUp Pro yesterday. I started to create a popup and tried it out a couple of times using the Preview button and it worked fine. Now when I click Preview, it won't display and I just get the spinning grey disc as if it's trying to load but can't. The only way to get rid of the disc is to reload the page which takes me back to the Edit popup page.