Hi, my setup is:

Nginx 1.10
Php 5.5
Mysql 5.5

Joomla 3.6.0
K2 2.7.0
Simple Image Gallery Pro v3.1.0
Frontpage Slideshow v3.8.0

When enable https on my site appear 2 problems.

1.) Simple Image Gallery Pro: When try to add a new gallery dont see the text box to put name of gallery.

2.) Frontpage Slideshow: When try to add new slide dont see the k2 articles, just see only a white popup page.

Without https all work fine.

What can i do to solve this issue?

The solution:

Replace this => add_header X-Frame-Options DENY;

with this in nginx config file => add_header X-Frame-Options SAMEORIGIN;

Thank you for posting the solution Evaggelos :)