You probably arrived here because you came across an error message in the diagnostic tool.
Ajax is required for several functions, for example the group administration page. In contrast to normal web pages, the communication via Ajax can break already when your WordPress site displays a harmless notice that would otherwise disappear in the HTML code.
Find Out The Cause
Any plugin or theme installed on your web site can cause Ajax to break.
There are several things that you can try. Some of them just require you to be the site administrator, for others you need access to the WordPress files.
Click “Show the Response”
If you click on the button “Show the Response” on the “Troubleshooting” screen, you may see some text that comes before the {"data":"... part. Try to figure out, which theme or plugin has caused these messages. The plugin’s (or theme’s) name is often somewhere hidden in the path – look at the piece directly after “/plugins/” or “themes”.
Example:
Warning: Illegal string offset ‘ms_dos’ in /homepages/1/abc1234/htdocs/mystore/wp-content/plugins/hello-bully/class-admin.php on line 111Check the browser console
Open the JavaScript Console in your browser, reload that page that doesn’t work and check the error messages below for anything that gives a hint about the cause. If you see an error mentioning JSON, XML, Ajax etc., you could try to click there to see the response that came from the server. Often it contains some lines that shouldn’t be there.
Search for the breaking point
Deactivate all other plugins and switch to a default theme. Then run the test again (or reload the page where you encountered the problem). If now all seems OK, re-activate them one by one to identify the one where things break.
Enable debugging
Add to your wp-config.php file the following lines:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
Then run the test again (or reload the page where you encountered the problem) and afterwards check the content of the debug.log file in the wp-content folder of your site if there is any error message that matches the time when you ran the test.
(When you’re done, don’t forget to set the true values back to false, or just delete the lines. It is also recommended to delete the file debug.log since it might reveal information about your server.)
Fix The Problem
The issue can often be solved only by the author of the plugin or theme that has caused these messages. Alternatively you can hire a developer.
In some cases it may, however, already help to change the PHP version on your server, to turn off WordPress debugging or to update, deactivate or replace an outdated or faulty plugin or theme.