I have the same issue, but only when attempting to use the “Group” gallery. If I use independent galleries on individual pages, that works. But Group doesn’t work. The preview image doesn’t show, and if you click on any gallery you move to the secondary screen and sit watching the black icon with the never ending spinner. And if you use the “back” link, the gallery images are also just displaying the spinner.
If you refresh, then you regain the first image of the galleries, but never have I seen the preview image set for the Group. I’m sure it’s going to be a while before anybody tends to this post, so I guess using individual galleries is all I can do. I wonder if it will work to have multiple galleries on one page without being in a group….