The situation:

Instead to the "page_not_found.php" the server directs to a page with a Fatal Error message (example:http://www.olex.org/u.html)

This is what I did:

1. I have added the line "ErrorDocument 404 /page_not_found.php" into the .htaccess file and uploaded it in to the root directory

2. Within the folder "config" I have included the following line "$v->setThemeByPath('/page_not_found', "olex");" in to the "site_theme_paths.php"

Concrete5 is installed in the root directory. A developer tells me that due to the structure of our website it's not possible to have a custom 404 page - could this be true?

Any help is appreciated!

Thanks.
Alex