“Internal Server Error” while serializing JSON in Web API Post method

I have a server which communicates with client using ASP.NET Web API.

My model in server side is more detailed and client has less data so I use [DataContract]/[DataMember] to match the both.

Here is the model on the server side:

[DataContract]
public class Discussion
{
    public Discussion()
    {
        this.Posts = new HashSet<Post>();
        this.Members = new HashSet<UserProfile>();
    }

    [DataMember]
    public int DiscussionId { get; set; }

    [DataMember]
    public int AuthorId { get; set; }

    public int CategoryId { get; set; }

    [Required]
    [MinLength(4)]
    [MaxLength(140)]
    [DataMember]
    public string DiscussionSubject { get; set; }

    [Required]
    [MinLength(10)]
    [MaxLength(10000)]
    [DataMember]
    public string DiscussionDescription { get; set; }

    [Required]
    [DataMember]
    public DateTime CreateDate { get; set; }


    [Required]
    [DataMember]
    public DateTime LastChangeDate { get; set; }

    [NotMapped]
    [DataMember]
    public int NumberOfPosts { get { return Posts.Count; } }

    [DataMember]
    public virtual UserProfile Author { get; set; }

    public virtual Category Category { get; set; }

    public virtual ICollection<UserProfile> Members { get; set; }

    public virtual ICollection<Post> Posts { get; set; }
}

And this is the client side model:

[DataContract]
public class Discussion
{
    [DataMember]
    public int DiscussionId { get; set; }

    [DataMember]
    public int AuthorId { get; set; }

    [DataMember]
    public string DiscussionSubject { get; set; }

    [DataMember]
    public string DiscussionDescription { get; set; }

    [DataMember]
    public DateTime CreateDate { get; set; }

    [DataMember]
    public DateTime LastChangeDate { get; set; }

    [DataMember]
    public int NumberOfPosts { get; set; }

    [DataMember]
    public virtual UserProfile Author { get; set; }
}

Now when I send a post request, I get an Internal Server Error, and that even doesn't reach to the Post method point while I debug

Discussion discussion = new Discussion
{
    DiscussionSubject = subject,
    DiscussionDescription = description,
};

response = await client.PostAsJsonAsync(requestUri, discussion);
response.EnsureSuccessStatusCode();

I guess the problem is in deserialization because when I change the model to another, it is fine. This will be good to know that when I send a Get request instead, every thing is OK.

How can I solve this problem, and where can I see what is the internal server error exactly?

EDIT: this is the Post method in the server-side:

public HttpResponseMessage Post(Discussion discussion)

I Expect Deserialization happen automatically here (from received client Discussion to server Discussion).