Warning during plugin installation

Hello, I have recently add some plugins to my wordpress.
After install MyMail plugin I start to receive a warning (never received before MyMail installation).
Now, every plugin I try to install, return me the follow warning (w3-total-cache is a sample, it’s indifferent what kind of plugin I install):

Warning: ftp_rename() [function.ftp-rename]: /var/www/vhosts/****.it/httpdocs/wp-content/plugins/w3-total-cache/: No such file or directory in /var/www/vhosts/****.it/httpdocs/wp-admin/includes/class-wp-filesystem-ftpext.php on line 209
Plugin reactivated successfully.
Successfully installed the plugin W3 Total Cache 0.9.3.

All the plugins work without problems but I don’t know the origin of the warning.
Please, can anyone suggest me a fix?

Thank you