I like the new Gutenberg editor in WordPress 5, but the first time I used it, it simply wouldn’t save edits! I kept on getting ‘Updating failed’ message for the body of the edit (although editing SEO elements worked).
After some research, it turned out to be an issue with plugins like WordFence and WP Cerber, which block suspicious activity. To resolve without removing the plugin, go into the Hardening tab of WP Cerber and turn on the option ‘Allow access to the REST Api for logged in users’.