Unable to create a Credit Memo agaisnt an invoice with tax line after going live with R12 last week

AR Guru,

We went live with R12 (12.1.3) this past week. We are facing issue when our users trying to issue a Credit Memo against a transaction

That was created before we go live with R12 (transactions with transaction date of 11/21/2012 or older) and having TAX line.

We have no issue with issuing credit memo against transactions that were created with transaction date of 11/21/2012 or older when that transaction has NO TAX lines. 

Some notes about our R12 ebTax: We are NOT configured to do ebTax in our R12 . All our tax calculations happens out side of Oracle EBS R12.

Our invoices are imported to our AR application via AR Auto invoice. All the taxation already happened to our sales orders that comes to our AR application.