Migration to Big Sur has disabled security password

I recently updated my MacBook Pro to macOS Big Sur (from Catalina) and now am unable to unlock the Security and Preferences window to make any changes. I need to do this to allow my antivirus system access to specific files but can't now unlock these settings.

I've also tried on my wife's machine (who doesn't have any additional antivirus programme) and since update she also cannot unlock the Security & Preferences window (i.e. padlock will not unlock because it doesn't recognise the password).

Is this a common fault with the upgrade to Big Sur? How do we correct it? Any ideas / suggestions gratefully received.

Thanks.