Fatal error: Cannot declare class WP_REST_Navigation_Fallback_Controller, because the name is already in use in /home/customer/www/[your-domain-name]/public_html/wp-content/plugins/gutenberg/lib/experimental/class-wp-rest-navigation-fallback-controller.php on line 0
If you’re seeing the above error message on your website, the solution is fairly straightforward. You’ll need to access your host’s file manager and delete the Gutenberg plugin (you can reinstall it afterwards). Here are instructions for SiteGround, which you should be able to follow loosely on your own host.
In SiteGround, go to Websites and then Site Tools for your domain name.
Go to Website > File Manager
Inside the file manager, open the domain name’s folder and then go to public_html > wp-content > plugins
And then delete (or rename) the Gutenberg folder by clicking the three dots next to it.
Then, back on Site Tools go to Speed > Caching > Dynamic Cache
Click the broom icon to clear the cache. The error should be resolved!
If you were getting good use out of the Gutenberg plugin, you should be able to re-add it from Plugins > Add New with no problem.
We haven’t looked into why this happened, but it’s probably because something experimental in Gutenberg got pushed to WordPress 6.3, causing a conflict. The order of updates probably caused the issue.