SDL Tridion: How to resolve 'com.tridion.configuration.ConfigurationException: No RequestData for WebGUI' when enabling WebGUI monitoring in the cd_monitor_conf.xml?

While enabling the WebGUI monitoring in the cd_monitor_conf.xml of SDL Tridion 2011 SP1 HR2, I got the following error message: ERROR Agent - TMA-AG-30000 Failed to get Agent instance com.tridion.configuration.ConfigurationException: No RequestData for... Read the full text.