From shared hosting to dedicated server. Updating plugins problem

Hello everybody.

I’m not new to WordPress nor Linux, but I’m having some strange behaviours with my new server.

I’ve made a full (database + files) backup of my WordPress installation and a restore onto my new dedicated server.

Folder /var/www/html/

Permissions of /html/ and contained folders set to: 777 (for testing purposes). Safe mode Off.

WordPress works, but I have this problem:

When I try to upgrade a plugin WordPress asks me for FTP data; since the serve use sFTP protocol, I’ve added to my wp-config.php the line:

define('FS_METHOD', 'direct');

Now it doesn’t ask me for FTP credentials any more but the upgrade fails:

Downloading update from XXX
Unpacking the update.
Installing the latest version.
Removing the old version of the plugin.
Could not remove the old plugin.
Plugin upgrade Failed.

Any ideas?

Thank you