Unexpected problem 30006

Hello accounts-weldon-, many thanks for reaching out and welcome to the Community page,

We are sorry to hear you are receiving an unexpected problem 30006 error message. Let's get this all resolved for you. This error message tells me this is regarding linking credit notes to customers' invoices. In order for me to understand the issues further can you let me know if this is happening when you are trying to apply a credit note to a customer's account or payment? Or,  is this happening on just one customer or several customers in this instance?  Also could you please try some troubleshooting steps to see if any of these resolve the issue for you, can you please try clearing your cache and cookies, too all time, close the browser down, reload it and log back in and try again? If the issue still persists, can you please try a different web browser and device?   

We look forward to hearing back from you.