"Null" when you try to see picture 501 and up.
I reported this problem
My Photos Folder has 596 pictures.pdf
m in May last year too!!!!!!
I find this happens AS I'M UPLOADING new photos; and for a short time after they're done.
While I try to get in & make some fixes before they're released into the transcription queue, it's quite annoying.
Workaround: in the URL bar, you'll see a reference to "... = 500... " - which is the standard 500 photos/page.
If you manually edit that to 1000 or 2000 or whatever, you'll get more on that page.
This lets me make corrections to the typically 1,000-1,500 photos that I upload each time.