2.0: unexpected exception after Page_Unload()

[RAM]

Hello,
I am learning .NET 2.0 and I have written simple application with
Default.aspx page and logging in Logon.aspx/Logon.aspx.cs.
After Page_Unload of Logon.aspx.cs (I have checked by debugging) I
receive error that I don't understand:

Message: Exception of type 'System.Web.HttpUnhandledException' was
thrown.
Stack: at System.Web.UI.Page.HandleError(Exception e) at
System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest() at
System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) at
System.Web.UI.Page.ProcessRequest(HttpContext context) at
ASP.default_aspx.ProcessRequest(HttpContext context) in
c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET
Files\mimmagazyn\64a9e8a5\4b2f62e8\App_Web_bl9uersv.0.cs:line 0 at
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step,
Boolean& completedSynchronously)
Source: System.Web

There's no error in my code - I have checked by debugging.
Please help. I cannot continue working.
/RAM/