What does this exception mean?

[Greg]

In my ASP.NET 1.1 Web application I have an HTTP Module that is a "global
exception logger." that logs info about otherwise unhandled exceptions.

It has logged the following exception 4 times during the past couple of
months - and I have no idea what it means.

The curious part, to me, is the RAW URL. It is not something that I have set
in any links anywhere in the site. I'm wondering what's going on.

Here's some of the info logged by my global exception logger:
/Default.aspx?__EVENTTARGET=&__EVENTARGUMENT=&__VIEWSTATE=dDwtMjM5NTgxOTgwOzs+gtQlX8GJplxnKs1bJgUnSYlxTxs=&_ctl0__ctl0_menuCA_ContextData=/

Exception Message:
Exception of type System.Web.HttpUnhandledException was thrown

Stack Trace:
at System.Web.UI.Page.HandleError(Exception e) at
System.Web.UI.Page.ProcessRequestMain() at
System.Web.UI.Page.ProcessRequest() at
System.Web.UI.Page.ProcessRequest(HttpContext context) at
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously)

Inner Exception stack trace:
at System.Web.UI.Page.LoadPageStateFromPersistenceMedium() at
System.Web.UI.Page.LoadPageViewState() at
System.Web.UI.Page.ProcessRequestMain()

Inner Exception stack trace ("2nd level"):
at System.Convert.FromBase64String(String s) at
System.Web.UI.LosFormatter.Deserialize(String input) at
System.Web.UI.Page.LoadPageStateFromPersistenceMedium()

What's going on? any ideas?

Thanks!