Did you call the number the website gives you when you get an error? I had the same situation (booking a complex itinerary that won't be allowed after 10/6) and had luck with that number whereas I did not have any luck calling Mileage Plus. I've called it a couple times since to make adjustments to the reservation and while I haven't asked where they are, judging by accent and call quality, I believe I was talking with someone in the US in all three cases.

Looking at my call log, the number was 1-888-491-6573. Good luck! FWIW, the flights I couldn't book on the website priced correctly when I spoon fed them to an agent.