Threatened with legal action

Who exactly is Arrow Financial? Are they dealing directly with the company that you are paying off. I would have someone from there call them and tell them the situation.

do you still have that last bill?

if so, I would get an address for that law office, and then send them a COPY of that last statement, explaining that the debt collector informed you of this due date, and as such they cannot turn around now and claim that you did something wrong. you have it in writing that the next payment is due on that date, and if they want to sue you over that, it would look pretty damn ridiculous when you show a judge that they already gave you until 03/09 to make your next payment, and then they are trying to sue you for not paying on time. And i would tell this law office exactly that.

also, what law office are we talking about? A lot of these big debt collectors make claims that they are a law office, but it may just be a collector on the phone trying to scare you into paying. its always good to verify the identity of anyone calling you from a "law office", it is a violation of federal law(the fdcpa) for a debt collector to claim they are calling from a law office unless they really are.