The problem is the that get_current_screen is failing, but the plugin code hides the errors with the "@" operator.
See modules/wpl.m.post-type.wp-lead.php, lines 212 and 230.
First remove the @ sign, so the code reads as follows:
$screen = get_current_screen(); //@this function is not working on some wp installation. Look more into this.
Then, in wordpress-leads.php, insert a line to include the screen.php from the wordpress core. I've inserted it prior to the other includes, as follows:
include_once(ABSPATH . 'wp-admin/includes/screen.php'); // added by RAV / IGDM to fix screen error
That fixes it for me. Can't say I understand why the screen.php isn't loaded prior though.