Paid off account in June and got a mail moths later stating that account is outstanding

I cancelled my subscription with Chubb over 2 year ago and was contacted by NuDebt Management advising that there was money outstanding which needed to be paid. I made monthly payments and in June I got a confirmation from NuDebt advising that the account had been paid off. 3 months later they send me a 48 hour notice advising that there was still an amount outstanding and why I enquired why they had previously said it was paid off they said the client, Chubb, had given them the incorrect amount initially. Now they want to inconvenience me for their incompetence. Would never recommend anyone to use this provider