I set up a custom 404 page following what instructions were available. I put it in htdocs/errors and named it 404.php.
I used the Error Pages function in the control panel to route 404 errors to my custom page. I then tested it by entering a non-existent page.
After many hours of frustration, I found that:
Using the Error Page function in the Control Panel adds “http://” to the front of anything I type. To get around this, I edited the .htaccess file in htdocs.
Pointing to any pages in /errors always produces a different 404 page that appears to be from iFastNet. To get around this, I used a different directory, and now my custom 404 page is displayed.
I believe the reason this does not work is because you cannot have a directory or file named “errors” or “error”. If you try renaming the folder, does it work?