Strange, I booked LH712 premium eco and can't select a seat. On LH.com I get "Seat reservations are not allowed on this flight.". That is my return flight. Two weeks earlier I'll fly LH717 and I can select a seat. On EF I can see that the flight is basically empty

LH 712 0 FRA
11/13/15 6:00 PM ICN
11/14/15 12:05 PM 74H
Daily
63% / 20m F6 A1 J9 C9 D9 Z9 P9 G9 E9 N9 Y9 B9 M9 U9 H9 Q9 V9 W9 S9 T9 L9 K9

Does anyone know why this flight is so special? I think I'm booked into N class.

Thanks