It is very much browser based. IE on two different computers (work desktop, work laptop) has started erroring out as soon as I try selecting the first leg of a trip.

Doing the same search with the exact same parameters (origin, destination, dates, Economy, Unrestricted fares) in Chrome on the same computers makes it all the way through to payment and confirmation without issue. It would be irritating if United explicitly said they don't support IE, but it's infuriating that they don't say IE is unsupported...but they don't seem to be supporting using IE...