Comment 2 for bug 710214

I think the issue is that Drupal theme registry contains an entry for the ''nice_menu_primary_links'' linking it to the theme_nice_menu_primary_links. But this function (perhaps because it has been removed/renamed/moved in the latests nice_menu or because the nice_menu is not enabled) is not available when theme('nice_menu_primary_links', $primary_links) is called in template.php.

Did you try to clear the cache ?