I had a similar issue with this yesterday, I am using behringer ua202, and it has been working fine for months. I have not tried using the interface since upgrade to ios 8.1.x until yesterday.
I could not get cubasis to get input from the UA202, but other apps (ToneStack) picked up the audio no problem.
After some messing about i tried loading audiobus, I then loaded cubasis as the output device and then suddenly everything started working fine. And now the audio is being picked up by cubasis again, even without loading audiobus first.
Seems wierd, but audiobus seems to have fixed something. No idea what though ?