SQLite on Windows Phone 8.1 data contract

I'm developing some app on Windows Phone 8.1 Silverlight.

I have a problem with data contracts:

When I'm comming back to my app after for examlple invoke file picker, I'm getting unhandled exception:

"Error in line 1 position 229. Element 'http://ift.tt/1tk0a69' contains data of the 'http://ift.tt/1NZ9dzn' data contract. The deserializer has no knowledge of any type that maps to this contract. Add the type corresponding to 'Task' to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding it to the list of known types passed to DataContractSerializer."

My Task class:

[DataContract]
public class Task
{
    [DataMember]
    [SQLite.PrimaryKey, SQLite.AutoIncrement]
    public int IdTask { get; set; }

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

    [DataMember]
    public int? IdIssue { get; set; }

    [DataMember]
    public int? IdTaskGroup { get; set; }

    [DataMember]
    public long? IdPlannedRoute { get; set; }

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

    [DataMember]
    public int? IdOperatorPerformer { get; set; }

    [DataMember]
    public int _idTaskStatus { get; set; } //private

    [DataMember]
    public Microsoft.Xna.Framework.Color _color { get; set; } //priv

    [DataMember]
    public int IdTaskStatus
    {
        get { return _idTaskStatus; }
        set
        {
            _idTaskStatus = value;
            switch ((TaskStatus)value)
            {
                case TaskStatus.NotStarted:
                    {
                        this._color = Microsoft.Xna.Framework.Color.LightGray;
                        break;
                    }
                case TaskStatus.InProgress:
                    {
                        this._color = Microsoft.Xna.Framework.Color.Yellow;
                        break;
                    }
                case TaskStatus.WaitingForAcceptance:
                    {
                        this._color = Microsoft.Xna.Framework.Color.Blue;
                        break;
                    }
                case TaskStatus.FinishedSuccesfully:
                    {
                        this._color = Microsoft.Xna.Framework.Color.Green;
                        break;
                    }
                case TaskStatus.FinishedWithError:
                    {
                        this._color = Microsoft.Xna.Framework.Color.Red;
                        break;
                    }
                case TaskStatus.Planned:
                    {
                        this._color = Microsoft.Xna.Framework.Color.Brown;
                        break;
                    }
                case TaskStatus.Canceled:
                    {
                        this._color = Microsoft.Xna.Framework.Color.LightPink;
                        break;
                    }
                case TaskStatus.Suspended:
                    {
                        this._color = Microsoft.Xna.Framework.Color.LightCyan;
                        break;
                    }
            }
        }
    }

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

    [DataMember]
    public bool? Accepted { get; set; }

    [DataMember]
    public int? IdOperatorAccepted { get; set; }

    [DataMember]
    public DateTime? AcceptanceDate { get; set; }

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

    [DataMember]
    public DateTime? Deadline { get; set; }

    [DataMember]
    public long? IdLocation { get; set; }

    [DataMember]
    public long IdLocationCORE { get; set; }

    [DataMember]
    public int? IdActor { get; set; }

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

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

    [DataMember]
    public long? OperationCode { get; set; }

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

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

    [DataMember]
    public DateTime? ArrangeDate { get; set; }

    [DataMember]
    public int? ArrangeDateStartHour { get; set; }

    [DataMember]
    public int? ArrangeDateEndHour { get; set; }

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

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

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

    [DataMember]
    public string Title
    {
        get
        {
            return string.Format("ID: {0} Status: {1}", this.IdTask, this._statusText);
        }
        set { Title = value; }
    }

    [DataMember]
    public System.Windows.Visibility Visibility { get; set; }

    [DataMember]
    public string Status
    {
        get {
            return string.Format("#FF{0}{1}{2}", this._color.R.ToString("X2"), this._color.G.ToString("X2"), this._color.B.ToString("X2"));
        }
        set { Status = value; }
    }
    [DataMember]
    public Location _Location { get; set; }//priv

    [DataMember]
    public Location Location
    {
        get { return _Location; }
        set
        {
            if (value == null)
            {
                this.IdLocation = null;
                this._Location = null;
            }
            else
            {
                this.IdLocation = value.LocationID;
                this._Location = value;
            }
        }
    }



    public Task()
    {
    }

    public Task(int IdTask, int IdTaskType, int IdOperatorRegistering, int IdTaskStatus, DateTime CreationDate,
                int Priority, int IdDistributor, int? IdIssue = null, int? IdTaskGroup = null, long? IdPlannedRoute = null,
                int? IdOperatorPerformer = null, bool? Accepted = null, int? IdOperatorAccepted = null, DateTime? AcceptanceDate = null,
                string Notes = null, DateTime? Deadline = null, long? IdLocation = null, int? IdActor = null,
                string TopicNumber = null, long? OperationCode = null,
                Location Location = null, int? TaskFormType = null)
    {
        this.IdTask = IdTask;
        this.IdTaskType = IdTaskType;
        this.IdOperatorRegistering = IdOperatorRegistering;
        this.IdTaskStatus = IdTaskStatus;
        this.CreationDate = CreationDate;
        this.Priority = Priority;
        this.IdDistributor = IdDistributor;
        this.IdIssue = IdIssue;
        this.IdTaskGroup = IdTaskGroup;
        this.IdPlannedRoute = IdPlannedRoute;
        this.IdOperatorPerformer = IdOperatorPerformer;
        this.Accepted = Accepted;
        this.IdOperatorAccepted = IdOperatorAccepted;
        this.AcceptanceDate = AcceptanceDate;
        this.Notes = Notes;
        this.Deadline = Deadline;
        if (Location != null)
            this.Location = Location;
        else
            this.IdLocation = IdLocation;
        this.IdActor = IdActor;
        this.TopicNumber = TopicNumber;
        this.OperationCode = OperationCode;
        this.TaskFormType = TaskFormType.HasValue ? 0 : TaskFormType.Value;
    }
}

In my app I'm using SQlite for Windows Phone package library.

Any idea how to fix this problem? :)