You may need to uninstall the drivers.
Download this tool:
USBDeview
Extract the folder, inside that folder - right click the exe and Run as Admin.
Look over your list of USB drivers and delete any that say Nokia, Lumia or Windows Phone.
Plug the 950 back into the USB and let the drivers install.
FWIW, USB ports are overworked, the sheer number of drivers for devices we see on most computers leads to this occasional borking. I've used this method to solve issues for many devices including WP
and Android.
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