ACKERMANS *********** PAYMENT COLLECTIONS

I have an account with Shoecity/Ackermans/Tenacityinc, they claim I **** my monthly payment into an incorrect account (Nedbank - which they had been using since i opened the Ackermans account) and that they cannot find my payment. I make my payments using my mobile banking app, and the beneficiary for my payments has not changed ever since I started using their services. So I went to the bank and got proof of payment and sent it to them. I got a call from one of their escalations agents saying that I have made the payment into their Nedbank account which they are not using anymore since Nedbank is no longer their administrators. Now Ackermans are telling me they will not allocate my payment into my account with them because i paid it into the wrong account. Mind you I never received a notification saying that they had changed their banking details and my account with them is now in arrears. Instead their best advice is that I must run to Nedbank and get them to reverse the money (I do not even bank with Nedbank so they dont know what to do with me since I am 3rd party). At this point I do not know what to do and im afraid that my credit record will be negatively affected by this.