Verified it was installed on Special:Version. But when invoking Hover Gallery with "hovergallery maxhoverwidth=800 maxhoverheight=800" it just doesn't do anything. There is no popup.
Topic on Extension talk:HoverGallery/Flow
Thanks for the report. You're right, there was something broken. I just fixed it. Please download the latest development version 1.3 (master) and let me know of any more issues!