After I install the "world clock applet", I tried to add it onto the gnome applet, but I got following error message.

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The panel encountered a problem while loading "OAFIID:GNOME_WorldClockApplet".
Details: Failed to resolve, or extend '!prefs_key=/apps/panel/profiles/default/applets/applet_10/prefs;background=none:;orient=down;size=small;lock ed_down=false

Do you want to delete the applet from your configuration?
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Does anyone know how to solve this problem?
I am having the same problem with gWlan applet as well.