I’ve found the solution!
I was using “attach to post” button in the post editor to insert the gallery (<img/> format).
On most of the posts, it works.
On the other ones, it doesn’t (“Invalid displayed gallery”).
Where I was getting this error, I’ve replaced <img/> format with the shortcode to display thumbnails, so there it works.
But I still see this as a bug.
@photocrati If needed, I can send you additional data to help resolve the problem.
