This occurs when using Google Chrome and can be resolved by clearing the cache in the browser.
- Press Ctrl+Shift+Delete.
- Ensure that Time range is set to All time.
- Ensure the following are checked:
- Browsing history
- Cookies and other site data
- Cached images and files
If you are still unable to view the upload window, try using a different browser such as Firefox, Internet Explorer, or Safari.