In short, I try to select one of my uploaded images in my media library.
I click the image, hit the Set As Header button, then either Crop and Publish or Skip Cropping (it doesn't matter which), and then the main screen just goes white. Nothing actually happens. If I go back to the Appearance>Header screen, it's still set to whatever the default image is.
Pretty frustrating, it doesn't matter what theme I am using, I cannot select a header image from my Media Library. The only thing it accomplishes is making another copy of the image in the media library.