This error message is typically caused by unlinking and relinking a Stax Bill customer to a Salesforce Customer Account faster than the system has time to finalize the changes.
The error message indicates that the system is looking for a blank field, but the field already has a value in it. The field in question is the staxbillData.ID that is on the customer.
To fix this, manually remove the value in the staxbillData.ID field and link the customer again.