WordPress Toolkit displays an empty list of Wordpress installations (even the control bar is missing):
WordPress version is defined in
./wp-includes/version.php file as follows:
$wp_version = 3.9;
$wp_version = '3.9';
This is an internal issue with ID #PPPM-1752, which has been fixed in Plesk 12.0 microupdate #7.
Apply the latest Plesk microupdates.