property evaluation failed exception when logging into website. Not shown to user, but seen in the code behind

I am maintaining some forms authentication code. When people log into the website (login.aspx) it takes a very long time to
log in (20 seconds), and then they are finally redirected to a default.apx page indicating that they were succesfully
logged in.
 

Now I traced into the code to see why the authentication is taking so long. All I could see is that when the following
line of code is executed an exception is thrown.
             Response.Redirect(FormsAuthentication.GetRedirectUrl(txtUsername.Text, False)) ' Comment to self : Exception is thrown here

The exception message in the debugger in the catch statement says: property evaluation failed.
        Catch ex As Exception
            errorLabel.Text = "Error authenticating. ...