The error you are seeing is part of the new sandbox feature in WordPress 4.9, after a change is madewp tests to make sure your site isn’t whitescreened, if it can’t determine the state, the save is disabled to avoid accidental breakage.
What may cause the problem
If you use any services on your site such as load balancing, heavy caching, etc that may affect cookies being sent, as the error in question is related to the attempt to reach the admin area after a change, and if any of the data sent is being stripped by cache or loadbalancing services that would cause it to fail.
Additional troublesooting: Do you have any security plugins running, alternatively do you know if your host uses mod_security which is also known to cause some headaches?
Known Issues with Plugins
There are a variety of plugins reported to cause this issue, including:
- ShortPixel Optimizer (fixed with v4.8.4)
- WP Server Stats (fixed with v1.5.7)
- Custom Sidebars (working on an update)
- Magic Fields
- WP Spamshield
- Social Login WordPress Plugin – AccessPress Social Login Lite
- Ninja Popups for WordPress
- Custom firewalls or security rules
*This list was taken from WPengine.
Known Issues with Themes
There are also a few themes that cause this issue, including:
- Blogglo (fixed with v1.3)
- Custom Themes that call session_start() function unconditionally
For urgent changes, please follow the directions in the error message to deploy your changes over SFTPor git. To find out where the problem lies within the WordPress Admin Dashboard Editor, we suggest following these steps:
- Download the free Health Check plugin
- Make a backup of your website in User Portal
- Run a Health Check using the Health Check plugin. Look for any fields with a red “x.” You may see a message which reads: “Unable to perform a loopback request to your site, this may prevent WP_Cron from working, along with theme and plugin editors.”
- Copy your site to staging.
- Download the Twenty Seventeen theme and activate it. Run the Health Check again.
- Do you see a loopback or any other failed response? If so, disable all plugins (except Health Check) and run the Health Check again.
- You may see “The loopback request to your site completed successfully.” This indicates one of your plugins is the problem. Re-activate them one by one, running Health Check each time to isolate the problem. If you have a lot of plugins you can re-activate them in small groups instead.
- If you still see the loopback request error in Health Check, check with any external firewall settings or security settings in your .htaccess file.
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