Bad service from Nedbank

Bad service received from Nedbank, had a loan with Nedbank account number: *** which was paid off last year on the 25 September 2020 and got a settlement letter but now is get an SMS from Nedbank say I owe R779.58.

Contact number: *** ID number: *** Account number: *** Current loan details Loan amount R5 315.25 Outstanding balance R 0.00 Current interest rate 27,75% Settlement amount R 0.00