1) You are incorrectly updating the database without using the OVSD client or DataExchange.
2) There is a bug in the client where sometimes you enter a value in a field, then you press Cancel, but that cached value is not cleared out, so when you save the form, OVSD is trying to save invalid data. You might try to reproduce the error to see if this is the reason. In fact, we have only seen this problem after installing SP18, but we never had SP8.
I remember something about this. If you hide a required field of a form for a specific group (role), and if a member of that group tries to register a call using that form, user would be unable to enter information for that required field, and you may get that kind of error. As far as i remember you can disable required field warning in administration console.