I had an issue with a client site using Lighbox Plus running on Apache under SSL. The asset links (colorbox.css and jquery.colorbox-min.js) still had http in the url instead of https, causing some browser issues.
Changing line 41 in lightboxplus.php (V2.4.6) to the following fixed my problem.
$g_lightbox_plus_url = plugins_url('', __FILE__);
WP_PLUGIN_URL will not pick up on SSL and create asset links still using http, not https.