Did you ever get this figured out? I have the same thing happening.
I know it's not the bank, because I have an account there, and setup myself as a customer to test, and I can successfully accept ACH Echeck payments from my personal checking account, but a customer's account fails.
I spent 4 hours on the phone with support (the contact details link from Adrian_A's post takes you to a team that claims they cannot help. They told me to call the number on the back of my quickbooks debit card, which is a journey that I'd rather not explain, but I took the journey. That person then redirected back to a different phone number, where I eventually hit merchant services after navigating the phone tree).
I'd suggest going to Merchant Services first, although they also claim that they cannot help. Told me that there is a mysterious (and apparently uncontrollable) algorithm that flags some accounts, and if it's flagged, then I should wait "at least 2 weeks" before I try using that account again. There's apparently no way to override this according to the rep that I talked to, which seems absurd because if my AMEX has a transaction that they think is fraudulent, I get an immediate text, to which I can reply and say "yes this is me" and it'll go through.
New to the quickbooks world, but I've been around software for a very long time. The hoops and hurdles for support are unreal, the error messages are unactionable as an end user, as for this particular situation the lack of a believable answer to something that obviously isn't a "mysterious algorithm that sporadically flags things with no possible corrective action" is an interesting way to try to close a support case.
