[PopUp Pro] Popup pro no longer working

Popup Pro again has stopped working for this site (GerryMorrisonPhotography.com). The popup preview works fine. When I click on the "purchase/enquiry" button (which should trigger the popup), it throws an SSL error that I do not know how to interpret.

I moved the site to a different host a few months ago and also installed an SSL certificate. The client didn't mention any problems until recently but as he doesn't look at his (hobby) site often, I suspect the problem has been there for some time.

I'm using Divi and that theme has been updated several times; I had to redo the button css in order to get it looking correctly so perhaps this is a conflict introduced by a theme update.

Support access has been granted for 5 days. Thank you for taking a look at this.

Diana