What is the problem?
In the Octoprint UI I get the message "Webcam stream not loaded" but the webcam is configured correctly, tests correctly in Settings, shows correctly in Octoclient, and even shows correctly if I click the URL (/webcam/stream/video.mjpeg) in the black error window.
I do have a bit of a complicated setup but this was working well until I changed to using the Raspi cam which is running under uv4l. I am NOT using the mjpegstreamer that is normally configured with Octoprint. This is uv4l which is a system service that is normally installed in Raspbian. And, as I say, the URL that is shown works fine if I use it in the same browser that is showing the Octoprint UI.
What did you already try to solve it?
I looked at everything I could think of.
/var/log/webcamd.log, syslog, dmesg, ... no logs, no support)
As far as I can tell there isn't a log which would show anything. There is nothing in octoprint.log.
Additional information about your setup (OctoPrint version, OctoPi version, ...)