Hi Ivan,
My understanding of the issue is the error page display but at the top of the page always contains [20-0004]. Correct me if that incorrect.
What is the web server vendor and version in your environment?
Is the custom error page unprotected?
The current redirect is to html page. If you try .txt file
ie:
ServerErrorFile="C:\error\error.txt"
Does it make any different?
Regards,
Kar Meng