I noticed that the pop up is not displayed if "Visit via a search engine" rule is activate and when a visitor comes from google. For bing and yahoo visitors everything works fine.
After changing the search engine array in popoverpublic.php the pop up started working for google visitors, too. I've changed 'images.google.' to 'google.' Now the array looks like this:
$SE = array('/search?', 'google.', 'web.info.com', 'search.', 'del.icio.us/search', 'soso.com', '/search/', '.yahoo.' );
Maybe this could help others who have the same problem..