I suggest you try resetting gnome-panel settings. The error occurs most probably because of conflicts in the configuration files of the various applets.
To reset gnome-panel settings, run this command:
rm -rf .gnome .gnome2 .gconf .gconfd .metacity All the command ever does is to recursively delete the files and folders in the above folders. These folders only contain configuration files and you don't have to worry about losing any data. The command does no obvious damage to your system or any data.