Harassed by FNB over paid account due to system error no one wants to investigate

If I could give a 0 or - star i would give it. I have been using FNB for more than 10 years and yes they have advanced access to their services but at this point they are too confident on their systems they would not listen to reason or try to help. I have a revolving facility account whis I pay every month and for some reason this month the account was paid the transactions shows that the amount was debited from my cheque on 24 August and 26 it reflected on the revolving facility and the interest was paid leaving the account with a positive balance. Yet FNB is so adamant that my account is in areas, I have been calling in, sending chats, emails asking for someone to help because I am being harassed by calls , sms and email threatening legal action yet my account is up to date. I would not wish FNB on anyone I know I am definitely changing banks, DO NOT GO FOR FNB BANK the worst.