Hi!
There is a bug in SIG Pro when Joomla SEO is enabled.
If an article is inside a section/category, URLs to it look like it was in a subfolder ( e.g. www.foo.com/folder/subfolder/article.html). That's when litebox doesn't find its close button. I haven't tried the other popup engines.

Solution: This can be corrected by taking variables fileLoadingImage and fileBottomNavCloseImage out of litebox.js, putting them in jwsigpro.php and adding JURI::base() in front of their path.

Another solution:
define a javascript variable with JURI::base() in jwsigpro.php and add that in front of the paths in litebox.js.

Nice report!

We will include it to the next "bug free" version!

Thank you my friend!