Autenticação com problemas

Olá! http://forum.imasters.com.br/public/style_emoticons/default/natal_noel.gif

Criei um sistema em C#, onde determino no web.config os usuário que terão acesso.

Basicamente, utilizei a autenticação 'windows', conforme abaixo:

<authentication mode="Windows"/>
 <authorization>
	<allow users="DOMINIO\user1"/>
	<allow users="DOMINIO\user2"/>
	<allow users="DOMINIO\user3"/>
 </authorization>
</authentication>

O user1 e user2 acessam o sistema perfeitamente. Já no user3, um erro aparece na tela.

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>

O que pode ser?