Hello,
clicking on add button the add popup-menu is hidden. I clear cache of browser and restart nginx.
Whats happend? How could i hunt the error?
Tell us which NC version and which browser (which extensions?) you use 
Does the browser console print anything or the nextcloud log?
Is the button not working in specific folders only or in all folders?
What did you change on your server right before that button menu started to become hidden?