Loss of Administrator access after upgrade to macOS Big Sur

 It appears that in some cases the upgrade from macOS Catalina to macOS Big Sur causes a user with Administrator access to lose these admin privileges. One of the main symptoms is the inability to unlock the Security and Privacy System Preferences app.

There is a thread on the Apple developper forum that describes the issue and provides possible fixes:

• Reset the SMC. This actually worked in my case, and I didn't try the other proposed fix.
• Reboot in Recovery Mode (press Command+R right after turning the computer on), open the Terminal (using the Tools menu) and delete /Volumes/Macintosh HD/var/db/.AppleSetupDone