This error occurs when posting an invoice from the Purchase Ledger batch screen and indicates an invalid character in the Invoice No. (also known as Transaction Ref.) field. This is often caused by pasting text into the field. Remove and re-enter the invoice number:
Click the three dots on the selected transaction in the Post Batch screen, then select Edit.
Amend the Invoice No. Field and re-enter it.
Click Save.
You will then be able to post the transaction.