This is an error caused by the Avatax bundle tha triggers when saving the customer record. This error occurs when a custom field we create on the customer record gets too much data in it. This happens when you map too many addresses to our entity/use codes.
The resolution is to go to the custom record AVAENTITYCODEMAPPING and delete at least one entry for the customer. The best way to exempt customers with a lot of addresses is to create an exemption certificate on the Admin Console or CertCapture, or uncheck the taxable checkbox on the customer record.