This exactly solved my problem. UE 4.10+DK2 was always OK. But UE 4.11+DK2 not…now…thanks to your help: UE 4.11+DK2 is OK!!
By the way: You can ignore the warnings that your DK2 may not be supported due to your “bad” computer. That is to say, your computer may not be strong enough for the Rift… but DK2 works fine for me, though the system check stated: USB and CPU not good enough.