3 answers
If you can view the setup on your SFDC or you should be able to find the Validation Rule that is giving the error message. If you can find it and copy&paste the VR formula we should be able to figure out why you're getting the error and how to fix it.
Here's more info => https://help.salesforce.com/htviewhelpdoc?id=fields_managing_field_validation.htm&language=en_US