Hi! I found a solutions for the same problem, but the cause was different for me. I was migrating my site to new server and in database (wp_options) was bad URL for “siteurl” and “home”. It needed to update to my current URL.
This post is 4months old, but i found a solution. First delete the gallery, than create a new one, and uncheck “Preview Photo via Page:” BEFORE saving the gallery.