"Another program has the camera"

The usual suspects

• The UseePlus app, if it is installed and running, even in the background or the menu bar.
• Another browser tab with the viewer open, or any other WebUSB page. Close the other tab, or press Release camera there.
• A native viewer or script you tried earlier (a Python driver, a libusb tool). Quit it.
• On Linux: the udev rule is missing, so it is not "busy" but "not allowed". Fix.

If nothing obvious has it

Unplug the camera, wait two seconds, plug it back in. The viewer notices the camera coming back and reconnects by itself once you have allowed it once.

Letting another program use it on purpose

Press Release camera in the viewer's toolbar. The page lets go of the device and shows "Camera released" until you press Reconnect camera. A recording in progress is stopped and kept.