I have a similar problem that i think is not mention in the bug report.
When i create a new site with "Own error documents" checked from the beginning, the "error" folder is created and it has the files in it, but when i try to access a non existent page on the site (to test the error documents), i'm getting the following:
The requested URL /test was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Thanks in advance.