[I find it hard to believe that they were not allowed to complete the flight as a one off waiver or did the extended flight at FL 270 mean they didn't have enough fuel to reach Warsaw.
It had nothing to do with altitude, Norwegian ATC simply refused to let them through even though they passed through busy airspace in US/Canada with inop transponder (on a waiver using special procedures). There was an interview today on Polish TV with the captain of this 787, he did not have high praise for Norwegian ATC.
I would imagine the 787 has 2 xpdrs, so apparently both failed
The problem was in the antenna itself, the problem was known at the time of (or shortly after) takeoff.