Well, for all intents and purposes you have (abandoned your new users). I recommended the purchase of your product to a client (now they have purchased). It plain doesn't work under any pop-up engine. Removing the additional js loads from your php file have no affect. New windows are always created when clicking on the thumbnails. Your free version worked fine. The paid version just doesn't. You have provided no solutions to the issue other than 'try another pop-up engine'.
This is abandoning your paid user base.
Why would I not create a negative comment on the extensions directory? I've been holding off a bit, because I know that once I do that I can't retract it, but your absence of help is leaving me little choice.

John

Had the same problem tried re-installing but still no show
Tried different Pop-up engines and all other trichs, had the GD 2 library etc

what solved it in the end for me is reducing the size of the images (I went back to about 25% of original size you have to check your own) there seems to be a internal server limitation which does not allow SIG to process the images to thumbnails if the files are too large.

Hope this helps