Accidental InvalidCastException

I get this exception in dictionaries and lists that have their own class. Example:

 List<DisplayAllQuestionsTable> dsa = (List<DisplayAllQuestionsTable>)Session["Display"];

Listing works 10-20 times when I use a session ... and then it starts throwing an exception. If I live on a PC for about 20-30 minutes .. I can run my web application as usual, and after running the code 20 times, it throws the same exception. Why is this happening?

Now I checked another simple code with Sesson:

public partial class Default2 : System.Web.UI.Page
{
    List<meem> moom = new List<meem>();
    protected void Page_Load(object sender, EventArgs e)
    {


       for (int i = 0; i < 20; i++)
            {
                meem m = new meem();
                m.i = i;
                moom.Add(m);
            }



       Session["meem"] = moom;
        Button ew = new Button();
        ew.Text = "Press me";
        ew.Click += Click;
        PlaceHolder1.Controls.Add(ew);
    }
    void Click(object sender, EventArgs e)
    {
        List<meem> moom = (List<meem>)Session["meem"];
        foreach (var item in moom)
        {
            Label l = new Label();
            l.Text = item.i.ToString();
            this.Controls.Add(l);
        }

    }



}

class meem
{
    public int i;
}

And it works 100%

Exception I get:

    Server Error in '/WebSite10' Application.
[A]System.Collections.Generic.List`1[DisplayAllQuestionsTable] cannot be cast to 
[B]System.Collections.Generic.List`1[DisplayAllQuestionsTable]. 
Type A originates from 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' 
    at location 'D:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll'. 
Type B originates from 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' 
   at location          'D:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll'.
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.InvalidCastException: [A]System.Collections.Generic.List`1[DisplayAllQuestionsTable] cannot be cast to [B]System.Collections.Generic.List`1[DisplayAllQuestionsTable]. Type A originates from 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'D:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll'. Type B originates from 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'D:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll'.