This happens when one of the Charge Card% fields on Transactions (i.e. Charge_Card_Alias, Charge_Card_Address_line) has a backslash (\) in it. This is usually found in the Charge Card Address field, but has also been seen in the Charge_Card_First_Name field.
Through the front-end, it is advised to review the Account's names and addresses and remove the offending character.
In order to correct Charge Processing, a back-end up update will need to be made to remove the offending character from one or more of the fields noted above.
Please create a support request at www.blackbaud.com/support
providing the error message and problem Account ID, if known, and referencing this solution.