See the edit below first, it now works!
I'm sorry, i don't have any good news
Rested BIOS (onboard sound disabled), re-installed Win7, installed all drivers, installed U46MK2 v3.0, installed Cross....same error.
except now it doesn't say that some channels are used by another application...
This is really getting on my nerves.
Could this be related to the new Audio-Device/Volume-Management in Windows 7? Could this thing called "Exklusiver Modus" in German - i guess "Exclusive mode" in English have any influence and Cross isn't given permission to access the U46MK2? - but disabling it didn't help either...just random thoughts.
The U46MK2 works fine with Winamp and Media Player Classic, i don't use other players.
Anyway, i'm out of ideas, i hope we'll find a solution
Greetings,
Markus
Edit:Well, its 6:35 in the morning and things finally seem to work
I've uninstalled the Mixvibes-Version of the driver and installed the ESI driver for the U46DJ (Revision B). Big thanks to Jim B for posting those links, i wouldn't have stumbled across the ESI driver again if he wouldn't have done that.

I don't know why exactly it works with this solution, but it works - and that's what matters.

Just to mention, i killed a few services in Windows also, don't know if that influenced it.
Thanks again for everyone's efforts
Kind regards,
Markus