Error saving or updating posts after upgrading to WordPress 5
Starting from version 5, the default editor in WordPress is changed to Gutenberg. When trying to publish a post using the new editor, the error messages “Publishing failed” or “Updating failed” is returned.
The new WordPress editor is utilizing the WordPress REST API. Using REST API option is turned off in uPress servers by default. In order to resolve the issue and turn the REST API on, go to security tab from the uPress panel, and activate the “Enable WordPress REST API” option