@Mr Y Were you able to figure this out? This just happened to me. I changed my bill pay to a different bank account. The payment went through with the proper account, but QB registered the bill payment with my previous bank account.

So now there is a discrepancy between my two bank accounts. And I cannot match the real transaction with the bill. I'm getting the same error message when I try to unmatch, delete, or change the bank info under "chart of accounts" since the payment has already process.