ASP.NET MVC WebApi & EF Lazy Loading = forever loading?

In my ASP.NET MVC WebApi project I have a Repository (with an Interface) that queries the DB via a simple Linq SingleOrDefault command and then outputs everything to The Controller. (EF = Database first against Oracle DB)

The Problem is that, when I turn on Lazy Loading (via the EDMX file property) the get is loading forever and there is no response. If I turn off Lazy Loading everything works fine?

Even stranger ist that when I put all the query in a console app and output everything via Console.WriteLine it works also with lazy loading?

Interface:

Public Interface IMedienRepository
    Function [Get](id As Integer) As MEDIEN
End Interface

Repository

Private db As EFEntities
Public Sub New()
    db = New EFEntities
End Sub

Public Function [Get](id As Integer) As MEDIEN Implements IMedienRepository.[Get]
    Dim _medien = db.MEDIEN.SingleOrDefault(Function(m) m.MEDIENNR = id)

    If _medien Is Nothing Then
        Throw New NotFoundException()
    End If

    Return _medien.SingleOrDefault()
End Function

Controller

Private _repository As IMedienRepository
Public Sub New()
    Me.New(New MedienRepository())
End Sub
Public Sub New(repository As IMedienRepository)
    _repository = repository
End Sub

Public Function GetValue(id As Integer) As MEDIEN
    Try            
        Return _repository.Get(id)
    Catch generatedExceptionName As NotFoundException
        Throw New HttpResponseException(New HttpResponseMessage() With { _
         .StatusCode = System.Net.HttpStatusCode.NotFound _
        })
    End Try
End Function

FYI: It isn't working even when I put the query directly into the controller!

I would be grateful if anybody knows about this issue and could help me because I like to use the Lazy loading feature in my MVC Views.

My work around for the Moment is to turn Lazy Loading off (db.ContextOptions.LazyLoadingEnabled = False) for calls via the WebApi ...