Cannot load content?

Hi, when I open the lightbox, some slides appear, but I keep getting the message “The requested content cannot be loaded. Try again later.” If I keep clicking through anyway, the next slide will eventually appear.

I’m using a standard WordPress gallery. One hint: when I turned on the Thumbnail view, it looks like there are lots of “empty” thumbnails (random amounts of space between the thumbnails with my images).

Any idea how I can fix this?

Thank you.