Don't know, all I can say is that when I must have had some problem at some time
at here what my web config looks like::
<!-- CUSTOM error MESSAGES
Set mode="on" or "remoteonly" to enable custom error messages, "off"
to disable. Add
<error> tags for each of the errors you want to handle. -->
<customErrors defaultRedirect="genericerror.htm" mode="RemoteOnly">
<!-- using mode="RemoteOnly" dishes out its own stuff -->
<error statusCode="404"
redirect="PageNotFound.htm"/>
<error statusCode="500"
redirect="InternalError.htm"/>
</customErrors>
Try it , see what happens ?