Thanks @Candy Luo for your feedback on this.
We're using Clearpass to authenticate. Here are the events from Microsoft\Windows\Wired-AutoConfig:
Issue starts with Event ID 14003 Source LanGPA not being able to apply the NAC client GPO:
A previously existing Wired Group Policy couldn't be applied to your computer.
Wired Group Policy Name: -
Reason Code: 2147943606
"netsh lan show interfaces" then shows as Status "Authentication failed." No wired connection possible anymore!
I found a workaround. Importing the Auth-profile XML via netsh lan add profile filename=<PathToProfile> interface=* seems to resolve the issue.
So I assume that the problem is client related and not server related. And it happpens only on 10% of all our clients being upgraded to 20H2. What is happening on the client that breaks the NAC configuration on some clients?