WordPress MU

Whenever I use "file_get_contents" or "fopen", with a URL, I get the following error:

[function.file-get-contents]: failed to open stream: HTTP request failed! HTTP/1.1 404 Not Found in /var/www/vhosts/example.net/httpdocs/wp-content/mu-plugins/mb_widgets/piwik_stats.php on line 86

The code used is the following:

ini_set ('allow_url_fopen', 1);
$fetched = file_get_contents($url);
//$fetched = fopen($url, 'r');

The URL used is correct.

I'm sure the problem has to do with the fact that I've got a subdomains installation of WPMU as on the same server a subfolder installation doesn't cause any trouble.

Anyone have an idea on how to fix this problem?

something wrong with your path. What's your WPMU installation directory?