I haven\'t been able to find a similar problem anywhere. Basically when I set the site to custom errors it displays the default 500 error message, however when I then set it to
IN IIS
Open the IIS7 manager
Select the Website and on its features view, double click on “Error Pages”.
Right click and select the “Edit Feature Settings…” or select the same from the Actions pane (in the right hand side)
Select the “Detailed errors” radio button and click on OK
Now, your client browsers will be able to see the detailed error messages.
ALSO...
Click on the website in content view. Go to "ASP" under the "Debugging Properties" make sure "Send errors to browser" is set to true.
IN CONTROL PANEL
Internet Options > Advanced (tab) > Un-tick "Show Friendly HTTP error messages"
By default IIS7 intercepts HTTP status codes such as 4xx and 5xx generated by applications further up the pipeline.
In web.config replace the line
<httpErrors errorMode="Detailed">
with
<httpErrors existingResponse="PassThrough" errorMode="Detailed">