You've got iCal set to either subscribe to or sync with a Yahoo calendar, and when it tries to access the calendar it is getting an error. Not sure if that "502" is a bad gateway error (network issue) or something Yahoo specific, but regardless, it's not being able to connect for whatever reason.
You can check the password and configuration to make sure everything is set up right on that calendar, or if you didn't realize you were subscribed to a calendar, delete it (iCal -> Preferences -> Accounts).