Décidément : il ne faut pas de chiffres dans le nom d'un fichier de log !

404.html

 <?php 
-spip_log('404 REQUEST_URI: '.$_SERVER['REQUEST_URI'].' | User agent: '.$_SERVER['HTTP_USER_AGENT'].' | Referer: '.$_SERVER['HTTP_REFERER'], '404erreur.'._LOG_INFO_IMPORTANTE);
+spip_log('404 REQUEST_URI: '.$_SERVER['REQUEST_URI'].' | User agent: '.$_SERVER['HTTP_USER_AGENT'].' | Referer: '.$_SERVER['HTTP_REFERER'], 'quatrecentquatre'._LOG_INFO_IMPORTANTE);
 header("HTTP/1.1 404 Not Found", true);
 ?>[(#HTTP_HEADER{"Cache-Control: no-store, no-cache, must-revalidate"}
 )][(#HTTP_HEADER{Pragma: no-cache}