Charing 47% interest monthly on settled account

This company wants me to pay interest on an account that was settled 2 months ago. I made a payment to the account from my bank's mobile app and sent them confirmation from my banking app that the payment was made - they refuse to accept this confirmation as it wasn't stamped by the bank. The bank won't give me a stamped confirmation without first concluding a formal investigation into the transaction, which apparently takes months. While most people happily accept the confirmations generated by banking apps, this company does not. I'm stuck between two companies because the one is telling me they want proof from the other, but the other is telling me to get the proof on their app, or wait, but meantime while I'm waiting, the other company is charging me interest at what I calculated as 47% annually - which is the whole reason I settled the account in the first place. My current bank charges 10.5% annually for loans. This is besides whatever my bank is going to charge me in admin fees for letting them run around and do frivolous investigations.