Please try to do the following:
- Update to the latest build of DSS.
- Try to use another computer to login to your WebGUI
- Try to use different internet browsers (ex: Firefox, Safari, Chrome, Opera,IE,etc..).
- Try to check your certificate, or remove it and put it again, see following link for details:
http://kb.open-e.com/Create-a-new-certificate_148.html
- Try to reboot.
One or more of the above steps should help you to fix this issue.