The default error page is:
Enterprise customers may upload a custom error page by contacting Incapsula support.
I dont like this, is there any way I can specify a unique URL instead of them seeing this?
This article covers the possible reasons for the error you provided above:
Is it possible to hide last octet of my origin server IP on such error page?
Is there a way to show custom error pages to our users based on the website we are protecting.