Similar issue: I successfully booked an itinerary on .bomb (ORD-LHR all on UA metal) about two weeks ago, but when I try to click into the itinerary in Reservations, I receive "ERROR! We were unable to retrieve the latest information for this itinerary."

The itinerary is retrievable on the app, so I am just assuming that it is some sort of back end website glitch, that the reservation is secure, and I plan to simply use the app instead of United.com to manage this particular reservation.