iMac camera problems with Studio display

Hi all,

We have an iMac 2020 running Big Sur and a recently acquired Apple Studio Display. We have found that the iMac's camera does not work when the Studio display is connected. Any apps trying to use it give an error saying 'camera not connected' or similar. Note the camera on the Studio Display is not found either. All returns to normal after the Studio Display is disconnected (though some things like Chrome seem to require relaunch or reboot.)

The Studio Display works ok otherwise, though it has an annoying habit of starting up with the display fully dimmed.

Thanks in advance for any help

Peter