Weird black square

Hi there, thank you for sharing your plugin, just what I needed!

The thing is, when opening the page that contains the shortcode, on the top of the A-Z index, appears a strange black diagonal square. See here: https://www.iustitia.com/wp/encyclopedia/

Do you know how to get rid of it?

Thanks in advance and keep up the good job!

Cheers.

The page I need help with: [log in to see the link]