gnome: Can't enter root password when using Gnome Software

When a user named 'admin' has been created and logged into Gnome, trying to install software with Gnome Software does not work. Using Gnome Software tries to gain administrative privileges and asks for a password. For this, it has somehow chosen the 'fbx' user account and asks to enter the password for that. This account is typically not used at all and the password is disabled by default. So, the user is stuck.