I am actually having the same problem on my
personal gallery. A few weeks ago I spent some time studying it and noticed there weren't any javascript or network errors, and I couldn't find any conceivable reason why it was happening. Unfortunately, I still don't know and I've had to move on to other things.
The popups are all Dialog controls by ComponentArt and I suspect the underlying cause is a bug or incompatibility with them. I am hoping to replace all ComponentArt widgets (dialogs, grids, menu, etc) with open source equivalents (most jQuery) for version 3.
For now, I'm not sure what to suggest other than to put on your troubleshooting hat and try to figure out what the heck is going on. I have the source code for ComponentArt so if I have to fix a bug and recompile, I am happy to do that.
Roger Martin
Creator and Lead Developer of Gallery Server Pro