Not sure if anyone else has encountered this?
When setting the following in the web.config
<httpErrors errorMode="Custom" defaultResponseMode="ExecuteURL" existingResponse="Replace">
We are not able to login to our admin. Say we login at 'yourwebsite.com/admin' it will not take us to the admin home page. But if you browse to yourwebsite.com/admin/default.aspx, it works.
If we replace the above code with the following:
<httpErrors errorMode="Detailed" defaultResponseMode="ExecuteURL" existingResponse="PassThrough">
We are then able to login to the admin side using yourwebsite.com/admin and this will take us to the admin home page.