Widget editing returns JSON response error

Question

I am trying to modify the sidebar widgets under Appearance -> Widgets, but whatever I do (just change one option, or remove a widget, etc.), as soon as I press the Update button, on the bottom left it says There was an error. The response is not a valid JSON response.

Upon inspecting the Network tab of the browser DevTools, the request is showing up as v1?_local=user and the request URL is https://host/wp-json/batch/v1?_locale=user with a method of OPTIONS. This request returns a status code 200, but there is no response data - unsure if this could be the issue or not.

What I tried so far (after each point I tried editing the widgets again):

• Updated WordPress to 5.9
• Updated all plugins
• Disabled all plugins
• Switched to a different theme
• Saved changes on the permalinks
• Used a different .htaccess suggested by a different website
• Checked the Site Health, it does not report any issues with the REST API

Everything else seems to work just fine, I can create/edit posts, pages, etc., but the widget editing keeps failing.

What else can I try to fix this issue?

Answer 1

If using IIS, check under "Handler Mappings" > "PHP_via_FastCGI" (or similar) > "Request Restrictions" > "Verbs" that "All verbs" is handled.

For me only "GET, HEAD, POST" was enabled and not OPTIONS.

Answer 2

The release of WordPress 5.8 removed the classic widgets editor in favor of the Gutenberg widget block editor to control your website widget areas.

References:

https://make.wordpress.org/core/2021/06/29/block-based-widgets-editor-in-wordpress-5-8/

https://wordpress.org/news/2021/07/tatum/

Old View of Widgets Dashboard

New View of Widgets Dashboard

To fix the issue add following code in the functions.php file.

File location: /var/www/html/yoursite/wp-includes/functions.php

function example_theme_support() {
    remove_theme_support( 'widgets-block-editor' );
}
add_action( 'after_setup_theme', 'example_theme_support' );

Then after reload the widgets dashboard, you will see the old dashboard view and will be able to edit the widgets.

OR

You may transform the existing widget as shown in the screenshot.

I hope these 2 solutions may help you.

Note: Remember to undo the changes done in the functions.php file.