aqui esta tu problema
Código c#:
Ver original....
<!-- AUTHENTICATION
This section sets the authentication policies of the application. Possible modes are "Windows", "Forms",
"Passport" and "None"
-->
<authentication mode="Forms">" -->" este cierre de comentario te sobra.
<forms name="SesionIniciada" loginUrl="Login.aspx" timeout="200"/>
</authentication>
...
quita eso y funcionara.