I'm having this same issue today
my last hard drive died so I had to get a new one and reinstall everything. IP cameras work fine but c920 wont work. It shows up in the list as a webcam but when I hit start, it just shows a gray screen
Camera works fine in the Logitech and Windows Camera apps.
I've tried all the USBs, ive tried uninstalling the drivers and letting them reinstall through Windows and downloading them through the Logitech website.
I've used iCamsource 2.9 and 2.9a
EDIT:
The last time I had this issue, I just had to plug a speaker in. No idea why but when I plugged in a speaker or earbuds, the cam would start but if I didn't have something plugged in, the camera would get some render error on iCam.
This time around, nothing is working
Not sure if it's a compatibility issue with the newest version of Windows 10