Fatal error: Call-time pass-by-reference has been removed in /var/www/html/wp-content/plugins/recent-custom-post-type/recent-custom-post-type.php on line 392
That line appears in the section labeled “PHP<5.3 compatibility layer.”
Removing the ampersand from $count in preg_replace() on that line seems to solve the problem and the plugin can then be activated without any errors. (Tested on PHP 5.4.10)
Also, “Recent Custom Post Types Widget” doesn’t appear in your “Select an Installed Product” dropdown menu in the support form. I selected “Recent Custom Posts” because that appears related.