USAA Trying to collect after 3 Years

I have settled a car loan with USAA in 2004 (the car was already repossessed at that time). We settled for about 60% of the outstanding balance, which I then paid off in full. They then send me a letter of settlement which stated:
"This will confirm that this account was settled in full on Feb 27 2004 for 5426,41. We will cease all collection activity and advise the credit reporting agencies that the account has been settled in full. The agencies will then update their records to reflect a zero balance on the trade line."

Now, I'm still banking with them for checking/savings. Last monday I tried to log myself into my online banking and couldn't log in - the message displayed that there was an outstanding balance and i needed to contact them at this number. When I called they informed me that they would like to claim the rest of the balance of the loan we settled 3 yrs ago, and that the bank had a write off/loss, which they would like to re-cover now, and that this is company policy. Also, I could not utilize the online banking features anymore until the balance was paid in full.
Then I get a letter yesterday from them saying that Payment is needed and that the debt remains unresolved. No amount, no account reference, nothing, just a blabla letter.

I'm under the impression I don't owe them a dime, just due to the letter they sent me after the settlement was completed. They keep claiming that they have full right to claim that money.

Any ideas on who's right would be greatly appreciated.

Thanks so much in advance - Dani