How to solve “Updating failed” with 403 Error when trying to save a Site or Post in Gutenberg?

There are many reasons this error can occur. Those two were the most frequent for me:

Session-Cookie

Open the Javascript Console by pressing F12 and click on the tab “Console”. If you see some error message that goes something like cookie_nonce it’s a session cookie problem.

The problem solves by just completely logging out and possibly also deleting all cookies of your website.

File-Permissions

If you have a webserver you manage on your own it may be that file permissions are wrong.

The instructions here have never failed to resolve my issues so far, so it’s worth checking out.

It’s those commands:

Add yourself to www-data group:

sudo gpasswd -a "$USER" www-data

Change file permissions:

sudo chown -R "$USER":www-data /var/www
find /var/www -type f -exec chmod 0660 {} \;
sudo find /var/www -type d -exec chmod 2770 {} \;