Unable to serialize the session state

When putting my application on a web server and trying to 'log in' I get the following error:

Server Error in '/' Application.
Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[SerializationException: Type 'Gebruiker' in Assembly 'App_Code.qzuhycmn, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.]
   System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type) +9452985
   System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type type, StreamingContext context) +247
   System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo() +160
   System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder) +218
   System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.Serialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder) +54
   System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck) +542
   System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck) +133
   System.Web.Util.AltSerialization.WriteValueToStream(Object value, BinaryWriter writer) +1708

[HttpException (0x80004005): Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.]
   System.Web.Util.AltSerialization.WriteValueToStream(Object value, BinaryWriter writer) +1793
   System.Web.SessionState.SessionStateItemCollection.WriteValueToStreamWithAssert(Object value, BinaryWriter writer) +34
   System.Web.SessionState.SessionStateItemCollection.Serialize(BinaryWriter writer) +638
   System.Web.SessionState.SessionStateUtility.Serialize(SessionStateStoreData item, Stream stream) +244
   System.Web.SessionState.SessionStateUtility.SerializeStoreData(SessionStateStoreData item, Int32 initialStreamSize, Byte[]& buf, Int32& length, Boolean compressionEnabled) +67
   System.Web.SessionState.OutOfProcSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem) +114
   System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs) +807
   System.Web.SessionState.SessionStateModule.OnEndRequest(Object source, EventArgs eventArgs) +184
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +148
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75


Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.1 

The only thing I do there with Sessions is this:

• Session["Rechten"] = "GlobaalAdmin"; (example)
• Session["gebruikerid"] = txtID.Text; (the ID they log in with)
• And redirect them to another page: Response.Redirect("LoggedIn.aspx",true);
• http://pastebin.com/jZprwA8m (Putting the gebruiker into a session)

There have been multiple topics about this problem but none of them seem to have helped me. Everything works locally, but online it doesn't.

Can we see the "Gebruiker" class? There error seems straightforward enough. Gebruiker is not attributed as being Serializable therefore it can't be placed in Session for StateServer and SQLServer modes.

EDIT:

After looking at the code you linked in, yes, it's probably because the class isn't serializable. The LINQ generated class should be a partial class so you can implement your own partial class that marks it as Serializable and the sessionstate requirements will be met then.

[Serializable()]
public partial class Gebruiker { //Lots of stuff }

EDIT 2:

Thomas, your Gebruiker class probably has a definition that looks like this right now (or something similar)

namespace XYZ
{
   public partial class Gebruiker
}

Just create another class file that has the same namespace and define the class with the Serializable attribute

namespace XYZ
{
  [Serializable()]
  public partial class Gebruiker{}
}

That's all you should need in this file. This way, the serializable attribute won't be overwritten when the LINQ generated Gebruiker class gets created.