Custom Forms Authentication in MVC 4, .Net 4.5 - asp.net-mvc

Question

I am posting this here after exhausting out every possible solution that could resolve the issue I am facing with my custom implementation of Forms Authentication. To give you a background of what I did so far... I was trying to implement the accepted solution from the following thread.. ASP.NET MVC - Set custom IIdentity or IPrincipal
So, I changed my Web.Config to allow for forms authentication. This redirects the user to the login page when the first request comes in. This is how my Web.Config looks right now.
<system.web>
<compilation debug="true" targetFramework="4.5" />
<httpRuntime targetFramework="4.5" />
<authentication mode="Forms">
<forms loginUrl="Account/login" timeout="30" slidingExpiration="true"></forms>
</authentication>
<httpModules>
<add name="ApplicationInsightsWebTracking" type="Microsoft.ApplicationInsights.Web.ApplicationInsightsHttpModule, Microsoft.AI.Web" />
</httpModules>
</system.web>
In the controller after the user is validated...
Dim serializer = New JavaScriptSerializer()
Dim serializableModel = New With {
.SchoolYearId = _userModel.SchoolYearId,
.AccessLevel = _userModel.AccessLevel,
.UserId = _userModel.UserId,
.FirstName = _userModel.FirstName,
.LastName = _userModel.LastName,
.SchoolYear = _userModel.SchoolYear,
.Role = _userModel.Role
}
'.Identity = _userModel.Identity
Dim userData As String = serializer.Serialize(serializableModel)
Dim authenticationTicket As FormsAuthenticationTicket = New FormsAuthenticationTicket(1, _userModel.UserId, DateTime.Now, DateTime.Now.AddMinutes(15), False, userData)
Dim encTicket As String = Security.FormsAuthentication.Encrypt(authenticationTicket)
Dim faCookie As HttpCookie = New HttpCookie(Security.FormsAuthentication.FormsCookieName, encTicket)
Response.Cookies.Add(faCookie)
Return RedirectToAction("Add")
Now in the global.asax file, in the post authenticate section..
Dim authCookie As HttpCookie = Request.Cookies(System.Web.Security.FormsAuthentication.FormsCookieName)
If (Not IsNothing(authCookie)) Then
Dim authTicket As FormsAuthenticationTicket = System.Web.Security.FormsAuthentication.Decrypt(authCookie.Value)
Dim serializer = New JavaScriptSerializer()
Dim deSerializedModel = serializer.Deserialize(Of UserModel)(authTicket.UserData)
Dim userModel As IUserModel = New UserModel
userModel.SchoolYear = deSerializedModel.SchoolYear
userModel.SchoolYearId = deSerializedModel.SchoolYearId
userModel.AccessLevel = deSerializedModel.AccessLevel
userModel.UserId = deSerializedModel.UserId
userModel.FirstName = deSerializedModel.FirstName
userModel.LastName = deSerializedModel.LastName
userModel.SchoolYear = deSerializedModel.SchoolYear
userModel.Role = deSerializedModel.Role
HttpContext.Current.User = userModel
End If
So, once the user the authenticated, the code goes through the postauthenticaterequest block and right after it exits the postauthenticaterequest block, the following error pops up which has been driving me crazy.
http://imgur.com/a/XGMnb
I did go through some of the problems that other users faced but this is something I couldnt find much help on. How do I go about solving this? This happens locally and not the webserver.