XML Serializer and Deserialize

[gplusplus]

I have wrote a program that can serialize and deserialize from xml to a class obj and vice versa. It works most of the time but every once in a while it bombs out giving me an exception that is "TypeLoadException" on this line...

 XmlSerializer xml_serializer = new XmlSerializer(typeof(UserPreferences));

its totally random when it happens and i have it set to retry if it fails and sometimes it retrys successfully and sometimes it just keeps failing. i would say i get this exception 5% of the time. i just want to know why its throwing this exception and howto fix THANKS ALL!!!

here is my userpref class

   [XmlRoot("Nelson")]
    public class UserPreferences
    {
        [XmlElement("properties")]
        public UserProperties properties;

        public UserPreferences()
        {
            this.properties = new UserProperties();
        }

        public UserPreferences(UserProperties properties)
        {
            this.properties = properties;
        }
    }
 
    public class UserProperties
    {
        [XmlElement("defaultSourceXML")]
        public string default_input_xml;
        
        [XmlElement("showFileSelection")]
        public string show_source_selection;
        
        [XmlElement("timeFormat")]
        public string time_format;
        
        [XmlElement("checkUpdates")]
        public string check_updates;

        public UserProperties()
        {
            this.default_input_xml = "None";
            this.show_source_selection = "False";
            this.time_format = "hh:mm t";
            this.check_updates = "False";
        }

        public UserProperties(string default_input_xml, string show_source_selection, string time_format, string check_updates)
        {
            this.default_input_xml = default_input_xml;
            this.show_source_selection = show_source_selection;
            this.time_format = time_format;
            this.check_updates = check_updates;
        }
    }