And please do another test to rule out the possibility (maybe unlikely, but who knows) that another app and/or Windows is trying to occupy Babyface's driver when SONAR is running:
In RME Driver Settings panel, click on WDM Devices
Configure and uncheck all WDM Devices listed.
Do this both for Stereo and Multi-Channel.
Only ASIO is now available for applications like SONAR (I assume you're using ASIO driver mode in SONAR; if not, please do so, at least for the test).
Edit to add:
You mentioned Windows System Sounds.
It's recommended to disable them, because Windows will use your device at any time a System Sound will occur and change the sample rate on the fly if necessary which could lead to problems if you use a different sample rate in SONAR.