I have installed the UV4L suite of software onto RPi3 with Jessie. I am using a USB cam (Logitech C170) rather than RPi cam. I have been struggling to get this working reliably. 

Initially I could not get anything working at port 8080. Finally I worked out (as it is not clear from the online instructions) that USB webcams trigger a streaming server at port 8090 NOT at port 8080. 

I rebuilt my Pi three times in order to test different approaches to getting this software working. With the most recent approach I avoided installing the auto start script..as I was suspecting some other background process was causing issues. When I manually start things after a reboot I get the issues below: 

    pi@PI3_TWO:~ $ uv4l --enable-server --device-path 001:004 --config-file /etc/uv4l/uv4l-uvc.conf
    <notice> [core] Trying to load driver 'uvc' from built-in drivers first...
    <warning> [core] Driver 'uvc' not found
    <notice> [core] Trying to load driver 'uvc' from external plug-in's instead...
    <notice> [driver] Device found at specified path
    <notice> [driver] Video functionality 'Webcam C170' recognized at 001:004
    <notice> [core] Device detected!
    <warning> [core] Cannot create /dev/video0 because file already exists
    <notice> [core] Registering device node /dev/video1

Why would it be not initially finding the uvc driver, and what is external plugins? Why is there already a video0 file...and is this a problem?

But more importantly, 
The streaming server displays, and I have it configured for https. It displays the MJPEG stream fine. But the display will not work with the WebRTC or conerencing options. I just see a black screen. I want to get this working. I have enabled the major WebRTC options in the config, but it still doesnt work. How can I debug / get the camera working with the WebRTC and conference options?