آقا به جای

<!--
The <customErrors> section enables configuration
of what to do if/when an unhandled error occurs
during the execution of a request. Specifically,
it enables developers to configure html error pages
to be displayed in place of a error stack trace.

<customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
<error statusCode="403" redirect="NoAccess.htm" />
<error statusCode="404" redirect="FileNotFound.htm" />
</customErrors>
-->

اینو بزار توی web.config

<customErrors mode="Off" defaultRedirect="mycustompage.htm"">
</customErrors>

با این کار تازه میتونی علت اصلی خطارو ببینی