Error de web.Config

Porque una aplicacion que se subio a un servidor web para ser ejecutada
por
la internet , saca frecuentemente error de web.config

Server Error in '/' Application.

Runtime Error
Description: An application error occurred on the server. The current
custom
error settings for this application prevent the details of the application
error from being viewed remotely (for security reasons). It could,
however,
be viewed by browsers running on the local server machine.

Details: To enable the details of this specific error message to be
viewable
on remote machines, please create a <customErrors> tag within a
"web.config"
configuration file located in the root directory of the current web
application. This <customErrors> tag should then have its "mode" attribute
set to "Off".


<!-- Web.Config Configuration File -->

<configuration>
<system.web>
<customErrors mode="Off"/>
</system.web>
</configuration>


Notes: The current error page you are seeing can be replaced by a custom
error page by modifying the "defaultRedirect" attribute of the
application's
<customErrors> configuration tag to point to a custom error page URL.


<!-- Web.Config Configuration File -->

<configuration>
<system.web>
<customErrors mode="RemoteOnly"
defaultRedirect="mycustompage.htm"/>
</system.web>
</configuration>

he tratado de cambiar la linea 41 que es donde los proveedores de hosting
dicen que existe el error pero la aplicacion sigue sin funcionar, ni
siquiera
linkea la primera pagina