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