Finally SOLVED! I still have no idea where the configurations is stored, but it can actually be changed very easily by the VNC control panel accessible on the desktop, via the small VNC applet on the right top of the screen. When the "VNC Server" window opens, click the menu button in the right top corner of the window and select "Options" from the menu. It will ask for your user's password and then open the Options dialog. Select "Expert" panel on the left and then find "Authentication" in the list of parameters on the right. Set the value manually to "None" and confirm that you are sure you really want to disable the authentication. Click "Apply" or "OK" and you are done.

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