Hi. I have the same error, but here you can see could be necessary to update a config file on your webserver.
“To tell your web server to use your custom error files, you’ll need to edit the .htaccess file in the main directory (where main index.php file resides) of your WordPress installation. If you don’t have an .htaccess file, see Editing Rewrite Rules (.htaccess) on how to create an .htaccess file.
To ensure the server finds your 404 page, add the following line to your .htaccess file:
ErrorDocument 404 /index.php?error=404″
I have no access to my server, so I have anything to do. I hope this was usefull.
Bye
Pol
