Back end menu Items not expanding/collapsing

I recently installed a WordPress instance inside my site and in this instance, a few weird thing are happening with the back end interface:

1- When hovering over the menu items, the subcategories don’t show up. To access “Themes”, for instance, I have to click on “Appearance”, wait for the page to load showing the submenu items, and only then click on “Themes”;
2- When editing menus, there is an accordion on the left with the items that can be added to the menu. In my case, when the page loads, only the “Pages” item is expanded, and others like “Links”, “Category” and “Products” are collapsed. The problem is that I can’t expand them. When I click on them, nothing happens, making it impossible to add Categories to my Menus;
3- The same thing above happens with the “Screen Options” and “Help” menus on the top;
4- When selecting items on a list (products, categories, etc), the box on the top that selects/deselects all the items simply does not work. It gets ticked but does not tick the boxes below.

I have already deactivated ALL my plugins, so I doubt it could be a conflict. Also, this has happened since the fresh installation of WordPress.
Tried also accessing it from different browsers…same issue.
When I open my other WP Admin on the same domain, I don’t have these issues…it is exclusive to this new instance.