Outlook Calendar "Server responded with an error" message

Help, can't figure out to get out of this. My only options are to 1) Revert to server (which is highlighted but won't go anywhere when clicked), 2) Try Again 3) Ignore (last two are shaded). I've tried force quitting Outlook Calendar and rebooting entire computer but the error message appears when it's re-opened. I even went to my Iphone and tried deleting any calendar event on July 17 (all years), in case that would help. Nothing's helping. Can someone please provide steps to be able to clear this and access my Calendar again?