I can not install anything through 'Ubuntu Software Center'. The system refuse to accept the su password that it accepts in terminal. It might be relevant that I've had change the user's password a while ago but do not remember the old password. Also I have added the user as a sudoer in /etc/sudoers.
Please notice that I have this authentication problem only in Gnome, where I get
Your authentication attempt was unsuccessful. Please try again.
whenever I enter my user password in the authentication window. I can use the very same password of the same user to install the package from terminal. So basically I'm limited to terminal for all my root activity.
How can I resolve this annoying problem?