PHP Error – realpath(): open_basedir restriction in effect.

Anyone having this issue?

PHP Warning: realpath(): open_basedir restriction in effect.

File(/var/www/vhosts) is not within the allowed path(s): (/var/www/vhosts/domain.com/:/tmp/) in /var/www/vhosts/domain.com/httpdocs/wp-content/plugins/w3-total-cache/inc/define.php on line 749

Is this a bug? Where is realpath() defined?

http://wordpress.org/plugins/w3-total-cache/