Dashboard buttons not working

I’m editing a website and some buttons in the edit page section don’t work. The collapse buttons on the top of each section on the right side don’t work aswell as the functionality to reorder them. I need to edit a gallery which is accessible by one of this side section but when I click it nothing happens.

I opened dev tools and I get these errors:

1. “SyntaxError: Unexpected end of JSON input” 2. “Cannot read property ‘hasClass’ of undefined” (This one appears multiple times) 

And the error comes from these lines:

1. “<!DOCTYPE html>” at post.php?post=256&action=edit&lang=es:1 2. “s.hasClass(“hidden”)” at wp-auth-check.min.js?ver=5.5.3:2 

What can I do?