If you are getting the error whilst trying to post a invoice from the Purchase Ledger batch screen, then this is due to an invalid character in the Invoice No. (aka Transaction Ref.) field, potentially caused by text being pasted into the field.
To correct it you will need to do the following:
Click on the three dots on the selected transaction in the Post Batch screen and then select Edit
Amend the Invoice No. Field and re-enter it.
Press Save
You will then be able to post the transaction.