Somehow I believe this is connected to the new M1 Macbooks and how bigSur is optimised foi their improved battery capabilities...

Same here with my 3 external monitors: 1 USB-C to HDMI 1 USB-C to HDMI 1 adapter USB-C to HDMI + cable HDMI (Not working). Everything was fine till the update. Now I get this "USB accessories deactivated" as soon as I plug the HDMI, even if alone.