Just to make sure:
Check the date and time in the clock on your computer: (double) click the clock icon on the Windows Taskbar.
If you can't add an exception, but get a pop-up with the error message then you can check the pref browser.xul.error_pages.enabled on the about :config page and make sure that the value is set to the true (default).
Also make sure that you are not in (permanent) Private Browsing mode.