Hi John,
as you wrote, the Social Billing program does what it is supposed to do. The error occurs in the Invoicing process. SAP Social Services is using the PSCD standard invoicing process - there is nothing social specific in the invoicing process.
Working in the area of Social Services, I can only recommend to open an OSS-message. I never heard of such an error before.
Sorry for not being able to give a better advice and best regards,
Roland