Are the current date (including the year) and time shown on your system clock? If not, correct them and try again.
Otherwise, launch the Activity Monitor application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ If you’re running Mac OS X 10.7 or later, open LaunchPad. Click Utilities, then Activity Monitor in the page that opens.
Select All Processes from the menu in the toolbar, if not already selected. Enter "ocspd" (without the quotes) in the "Filter" text field. Is a process with that name listed?
If not, select Go ▹ Go to Folder… from the Finder menu bar. In the text box that opens, enter
From the folder that opens, move these two files to the Trash:
You’ll be prompted for your administrator password when you do this. Then reboot, empty the Trash, and try again.
[If this doesn’t work, it’s something like “LittleSnitch” or “Hands Off.” Test in safe mode.]