Added [Serializable()] to my DTO and now getting an error with deserialization

Question

Working on moving one of our projects from inproc to using SOSS and ran into some errors that indicated some of our models/dtos needed to be serializable. For all but one of these, adding

[Serializable()]

to the top of the class resolved all issues. For the remaining one, it is now returning this error:

System.Exception: An service exception has occured the exception was System.ServiceModel.Dispatcher.NetDispatcherFaultException: The formatter threw an exception while trying to deserialize the message: There was an error while trying to deserialize parameter http://tempuri.org/:GetAllAttachmentsResult. The InnerException message was 'Error in line 1 position 686. 'EndElement' 'YourDTO' from namespace 'http://schemas.datacontract.org/2004/07/Business.Objects.Message' is not expected. Expecting element '_x003C_AttachmentDate_x003E_k__BackingField'.'. Please see InnerException for more details. ---> System.Runtime.Serialization.SerializationException: Error in line 1 position 686. 'EndElement' 'YourDTO' from namespace 'http://schemas.datacontract.org/2004/07/Business.Objects.Message' is not expected. Expecting element '_x003C_AttachmentDate_x003E_k__BackingField'.

Here is the DTO:

    namespace Business.Objects.Message
{
    [Serializable()]
    public class YourDTO
    {
       public long Id { get; set; }
       public FileType Type{ get; set; }
       public string Name { get; set; }
       public DateTime AttachmentDate { get; set; }
       public string UserName { get; set; }

   
   public YourDTO()
   {
       Id = 0;
       Name = string.Empty;
       Type = FileType.Unknown;
       AttachmentDate = DateTime.MinValue;
       UserName = string.Empty;

   }
}
}

The code that uses this simply populates the DTO from a db as a DataTable and then sends back the list of DTOs after mapping.

Couple things to note:

1. This DTO is passed through a nuget package to the solution in question.
2. I did try [DataContract] as well but this did not work either.
3. This has worked for years up to this point. Only change is adding SOSS to web.config for SessionState and the [Serializable() attribute addition.

Any ideas would be greatly appreciated!