I had the same problem with my website. The problem was one of the plugins. It was the plugin “PayPal Plus for WooCommerce” by Inpsyde GmbH. I’m not sure if it was because they haven’t updated their API, JSON, or AJAX code after the last WP update, but I flushed the permalinks twice and went through each plugin by deactivating them one by one, checking the functionality of the “new post” and category feature each time, and then voila that was the problem.
