"There was no response from your device while checking for out of date Appointments. Disconnect your device and try again. If the problem persists, reset your device according to your manufacturers documentation and synchronize again."
Normally if you get this error if you go to Activesync and change the rules to "replace what is on your device" then sync, it kills that error. You can then go back and reset your rules to the way they were. In effect you are finding and removing that one email, or file that is causing the error.