If the check paid an invoice previously entered (debiting expense and crediting payables), then the "expense" side of the transaction would be debiting the payables account, not an expense account.

Common causes of this error are:
1. A credit note or debit note or adjustment was entered with a distribution to the payables account. This would result in a change in AP, but the transaction would net to zero in GL.
2. An adjustment was entered directly in GL to the payables account.