WP 2.5 Uploader Suddenly Asking User to Login

For the first few weeks, all my installations of WordPress 2.5 worked completely perfectly. Suddenly, however, in the last week or so, the uploaders for every instance will crunch like they’re uploading files and then go to the login screen in the uploader window. If you try to login (even though you’re obviously already logged in), it will then produce a 404 error inside the uploader window. This clearly seems to be a new issue with my server/webhost. Any ideas as to what setting on my account may have changed to cause this strange behavior? I know it’s not a javascript or a Flash issue, particularly because, as I said, it was working originally. Any tips and pointers that I can take to my webhost’s tech support people would be greatly appreciated.