Working with a fresh install of WordPress 2.5, I upload a jpg and it crunches properly, but instead of giving me the option to insert the image into my post, it bumps me immediately to the admin login screen.
When I login, I get an 'Error 404' message. When I check the collection of images later, the image isn't there.
I checked and didn't see this problem listed. If it has been covered, I apologize.