Hi all.
as you can see in http://www.gotjeep.net/blogs/ it's a kind of Microsoft BUG.
so the workaroud is:
ScriptManager.RegisterStartupScript(Me.UpdatePanel1, "string".GetType(), "alertMessageErrorScript", "alert('this is an error message error');", True)
well
maxsoft
happy code!
remember... happyness it's workaround your pc :)