Image upload frame becomes a 404 after uploading

On a blog I’m working on, when I upload an image to the server with the Upload area in the Write Post page, the frame that the Upload area is in becomes a Page not found page. But the image does get uploaded successfully, because when I go to the Browse Files tab in the Upload area, the uploaded images appear. Not sure what’s going on, but the Page not found page is misleading… any way I can fix this?