Why is Divi giving an error “your save has failed” and opening a preview of the page in a new tab?

The reason may be that you have a Wordfence installed, and your page has a Code module, which may be causing Wordfence to clamp down on your page editing experience.

The solution can be to put Wordfence into Learning mode.

Go to Wordfence > All Options > Firewall Options > Basic Firewall Options >Web Application Firewall Status > changed “Enabled and Protecting” to “Learning Mode”.

Wordfence will now learn that web pages with a Divi code module are okay to save properly.