Error Evaluating CHECK constraint.

This database (Access 2003, Windows XP) has been operational for 2 months now. We enter data into the database using a form. Recently we have been unable to add new records to the database and get this error message each time we try to save a new record: "Error 'Error evaluating CHECK constraint. 'in the validation rule". This is a multi-user database so no validation rules were built to prevent this very issue. I have check all the fields in the table to make sure that no one has inadvertently added validation rules. I have not found any validation rules that may have been added by mistake. I have turned error checking off/on and off again but this does not seem to help.

Does anyone know how to fix this problem? Please respond.

Thank you.