I'm encountering a display problem with popup pro on IE 11 - no troubles on FF and Chrome : the css styles don't load correctly on first display. The trouble disappears when you double click on navigator's window... awesome ! ^
The main default is the position of the popup feature image : to ensure a correct display, i used customs css rules, in particular through a "position: absolute !important" applied to ".wdpu-image img", and others rules but less importants. This is of course a temporary solution.
I've found errors - via IE debogger - coming from wpmu-ui.3.min.js. These defaults come probably and partially from IE, but can you please inform developpers in order to improve popup pro plugin.
I opened the support access on dev site, but you'll need also other informations to pass through htaccess control : please tell me how to send you these informations in private way.
I'd like to have a wonderful day !