If you receive this error in any area of the app, the first thing you'll want to try is a different browser. If you do not have the same problem in another browser, refresh the cache of your main browser and check again.
If that doesn't solve your problem, check to ensure that the gallery embed code is correct in the theme. 99% of these errors are due to a user not updating the gallery code when they create a new gallery.
If you're still having issues, click the Help button in the lower right of this page and send us an email with screenshots and links so we can expedite your support.