If a C appears next to a transaction, this means that it is 'completely settled by post-dated transactions'. This text should appear at the bottom of the report.
For example, if you have an invoice on Sept 1 but the payment is dated Nov 1, then running that invoice will have a C next to it, meaning that the amount that pays that invoice is dated in the future. Also, if you backdate the Report Date, this affects which transactions are selected.
Try previewing the report with the 'Applied Details' option selected to see if this sheds any light on things.
Edit or View the vendor and go to the Transactions tab. The transactions that appear on the Open Payables report appear on the Current screen. You probably will be able to find the invoice(s) in question; highlight and look at the Applied details. What is the date of the payment (or other applied details) in the grid halfway down the screen?