'You are not authorized to access this page' error when importing full page of images

Unfortunately, I don't have an exact number of images where this fails for me, but here's the best I got.

I had several hundred images to import. I stuck them in the right directory. The image import page shows them just fine. I found that if I clicked the 'select all' checkbox, chose a gallery, and then attempted to import, it would give me the 'You are not authorized to access this page' message.

I tried importing 1, 5, 10, 15, 20, 25, 30, 40, 45 at a time - all of these numbers imported fine. If I *manually* checked off all 50, or 49, I got the same error. I was trying to narrow down to what exact number it fails on, but I ran out of pictures I needed to import :)

Attached is the set of errors I got while trying to import bigger groups of images.