If you see the old unlock screen (gray window asking for your username and password) you have to delete the following file:
/etc/cacloginconfig.plist
If you delete this file, you will see the new unlock screen (like the login screen) and touch ID will work again.
Summary: If "/etc/cacloginconfig.plist" exists in 10.9 +, the screensaver unlock screen will be controlled by SecurityAgent (old style). Admins, when they successfully authenticate, will be allowed to take over the user's session. Even if "system.login.screensaver" in AuthDB is set to "use-login-window-ui", this will be overridden by the existence of "/etc/cacloginconfig.plist". We will still be presented with the SecurityAgent style login window.
More Info:
http://www.openradar.me/19177988
The App can have read/write access to a location if it gets your permission using an entitlement as part of the App Sandbox, in other words this is how its suppose to work. The App is code-signed and trusted, its asking your permission, in theory all should be good.
As noted in the Mac Technology overview:
App Sandbox
Introduced in OS X v10.7, App Sandbox provides a last line of defense against stolen, corrupted, or deleted user data if malicious code exploits your app. App Sandbox also minimizes the damage from coding errors. Its strategy is twofold:
App Sandbox enables you to describe how your app interacts with the system. The system then grants your app only the access it needs to get its job done, and no more.
App Sandbox allows the user to transparently grant your app additional access by using Open and Save dialogs, drag and drop, and other familiar user interactions.
Specifically a developer can implement the following entitlement as noted here in Enabling App Sandbox, this is exactly what you described in your question, so this is probably the entitlement that the A/V program developer had utilized to obtain just read and write access.
com.apple.security.files.user-selected.read-write
Read/write access to files the user has selected using an Open or Save dialog
Note that this is not the same as true administrator user, since the program would not be granted the execute privilege.
Best Answer
Very similar error messages have been noticed by developers and users alike.
Check out this blog post. It's not an identical error message but perhaps this will fix it for you as well:
So, you might test this remedy by making a good backup of your Mac and ensure you either have the network passwords and VPN setup files you need (or can regenerate them / reset your WiFi routers to passwords you know) and delete the NetworkInterfaces.plist file.
If that fails to work, you may need to open a Mac App store support ticket and involve Apple in troubleshooting this further. If you can document the steps needed to reproduce this error, please post a follow up question.
Chances are there is a bug in some Mac App Store software that was downloaded onto your computer and getting Apple support involved mit fix it for you and others at the same time.