When I open the Minecraft Launcher it automatically opens a Microsoft window that says:
 
"Sign in with a security key
Your PC will open a security window. Follow the instructions there to sign in"
 
It doesn't open any other window for me to follow any instructions in order to enter a security key. This has become an issue since the latest update for Minecraft, previously could log in fine. Closing this window means I can log in to the launcher if I'm fast enough, but then I can see a button for the demo version, not the full version.

I use Windows 7 if that helps.