According to this article :

You can find the traffic that happens at the time you receive the 403 and whitelist that.

1. Try saving your post again
2. Go to WordFence
3. Click on Tools inside the WordFence menu
4. Click on Live Traffic. This view shows recent traffic to your site.
5. Filter the traffic to "Blocked By Firewall".
6. Click on the request you just made to save the page in your page builder -- it'll usually be the most recent request and from your general area
7. Double check that it was actually you making the request
8. Whitelist the request