Server Error in '/' Application

Bonjour
Impossible depuis quelques jours de me loguer à certains sites
Ma config : windows 7 pro 64 bits
Navigateurs : Firefox 11.0 & Internet Explorer
C'est la même erreur pour l'un et pour l'autre.

Donc quand je veux me loguer, j'ai la fenêtre suivante qui s'affiche :

Object reference not set to an instance of an object.

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.NullReferenceException: Object reference not set to an instance of an object.

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:

[NullReferenceException: Object reference not set to an instance of an object.]
M123.Web.Tchat.UI.ChargedLoginPage.SubmitImageButton_Click(Object sender, ImageClickEventArgs e) in C:\Sources\M123\Web\Chat\M123.Web.Chat\UI\ChargedLoginPage.cs:39
System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +108
System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +118
System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565


Version Information: Microsoft .NET Framework Version:2.0.50727.5448; ASP.NET Version:2.0.50727.5456