[NSFW] I cannot add new categories & set featured image

I was setting up the website when I realized I could not add new categories. I go to “/wp-admin/edit-tags.php?taxonomy=category” and I click on “Add new category” button but it does nothing.

After trying the following:

– change theme to see if the problem is the theme.
– deactivating all plugins to see if this resolves the problem.
– resetting the plugins folder by FTP.
– re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.

I do not know what else I can try to solve the problem.

Furthermore, I realized that the same thing happened to the other subdomains where I installed wordpress.

In the main domain WordPress is version 3.9, in the subdomains the version is 3.8.3.

Please helpme. If I don’t find a solution I have to change to an other CMS, and this is the worst-case scenario.

Thanks