Hi Prock, thank you for the tip. I’ll investigate the issue a bit further to see what’s going on here.
Just a follow-up for future reference, since I finally discovered what my problem was. It seems that, if you switch off your USB MIDI keyboard while Cubase is running, you will get the warnings about a missing device as soon as you try to load new projects. To avoid this, you simply have to restart Cubase, and the warnings won’t show anymore.
I think this is a snarky not silly comment in response to the post immediately preceding it. Your irony detector might need a tune-up. 
By the way this new USB support is not universal. C9 does not recognize my Behringer X-Touch if I turn it on after launching Cubase. I suspect this is the case for all devices that use Mackie Control. Can anyone confirm for other devices if this is true? Too bad because the X-Touch is where this capability would be most useful for me.
I have a Behringer USB board that doesn’t show up in the device manager. I am very frustrated with this Cubase 9. I have many different, older studio software and they all work with the Behringer mixer. I was duped into buying this piece of poop Steinberg crap. I wouldn’t recommend it to my worst enemy.
Why not just ask for some help with your issue? 
Because blaming s.o. is more simple.
First post as well.
My x-touches can be powered off and restarted - Cubase just needs to have the reset button in devices toggled and it sees them. They do get disconnected sometimes - especially when there is a lot of control changes from - for instance - my behringer bcr2000 - so the pnp support if highly valuable…