When using the 'unmapped transactions' setting and posting gifts to the General Ledger the error can occur if the gifts being posted have the GL Account number used in the 'unmapped transactions' setting.
The exception is a valid exception. Check to see if account number X has been mapped as the account number to use for unmapped transactions, as well check to see if account number X is used as the account number for the debit and credit account number in the transaction mapping. The system sees account number X and automatically assumes it is the unmapped account. The solution is to use a different account number for the unmapped transactions setting. The account number used should not be associated with any debit or credit account numbers. This could meaI there is a need to create a new account number to be used.