Workaround after encountering this problem: Restart the KDE policykit daemon:
$ /usr/lib/kde4/libexec/polkit-kde-authentication-agent-1
which is usually start by /usr/share/autostart/polkit-kde-authentication-agent-1.desktop (look into this file to see the command above).
This is also why logging out and logging back in again usually works.
Workaround after encountering this problem: Restart the KDE policykit daemon:
$ /usr/lib/ kde4/libexec/ polkit- kde-authenticat ion-agent- 1
which is usually start by /usr/share/ autostart/ polkit- kde-authenticat ion-agent- 1.desktop (look into this file to see the command above).
This is also why logging out and logging back in again usually works.