2 Logitech 930 USB cameras
1 Borescope USB camera
Server Error (resolved)
Many of the solutions posted here.
I was able to resolve my issues with the dreaded stream "server error" messages.
I am running four cameras on a PI4 4GB.
To resolve the issue I increased the number of buffers (NBUF) in each of the camera conf files to 10.
I set the output image format to JPEG for both of my Logitech 320s that have hardware mjpeg encoding.
I set the output image format to YUYV for my Raspi (PiCam) camera.
I set the output image format to YUYV for my USB Borescope camera.
I rebooted the Pi4.
This also results in quick connections to each of the cameras and drops the terminated streams quickly as the browser/client switches streams. Prior to doing this the streams stayed open causing increased network traffic until the browser/client exited the connection for many minutes
Note that you will have to check to see what output formats your cameras support and adjust accordingly.
Mjpeg encoding puts less of a load on the PI and USB.