I am getting fatal error on activate with the latest purchased version. I have tried on a couple diferent servers with same problem.

Fatal error: Uncaught Error: Call to undefined function WPF() in /hermes/bosnaweb28a/b903/dom.leslieiwg65520/public_html/wordpress/wp-content/plugins/wpforo-private-messages/includes/functions.php:41 Stack trace: #0 /hermes/bosnaweb28a/b903/dom.leslieiwg65520/public_html/wordpress/wp-content/plugins/wpforo-private-messages/includes/functions.php(34): wpforopm_activation() #1 /hermes/bosnaweb28a/b903/dom.leslieiwg65520/public_html/wordpress/wp-includes/class-wp-hook.php(303): do_wpforopm_activation('') #2 /hermes/bosnaweb28a/b903/dom.leslieiwg65520/public_html/wordpress/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters('', Array) #3 /hermes/bosnaweb28a/b903/dom.leslieiwg65520/public_html/wordpress/wp-includes/plugin.php(470): WP_Hook->do_action(Array) #4 /hermes/bosnaweb28a/b903/dom.leslieiwg65520/public_html/wordpress/wp-admin/plugins.php(193): do_action('activate_wpforo...') #5 {main} thrown in /hermes/bosnaweb28a/b903/dom.leslieiwg65520/public_html/wordpress/wp-content/plugins/wpforo-private-messages/includes/functions.php on line 41