TYPO3 Core

My bad, it checks for the TYPO3_REQUEST_HOST variable.

Some more info:
My TYPO3 install on localhost is in a subfolder, so I have http://localhost/sub1/, http://localhost/sub2/, etc.
When I set my error page to fileadmin/404.html, it always tells me that http://localhost/fileadmin/404.html doesn't exist, which is of course true.

That's because TYPO3_REQUEST_HOST is just the domain name, not the site url.

TYPO3 install on localhost is in a subfolder
$TYPO3_CONF_VARS['FE']['pageNotFound_handling'] = 'READFILE:fileadmin/notfound.html';
-> everything works as expected for me.

Still reproducable?
Realurl enabled?
OS/Webserver/PHP-Version?

Seems to work now. It's only been two major versions since :)

Resolved, not reproducable.

Thanks for the feedback!