The invalid JSON error is WordPress’s way of saying that there may be a more detailed error to be found in the Web Inspector, both in the JavaScript console or in the Network tab, when viewing the response from the server for the specific API request which attempted to load or save the information.
There has been a critical error on this website is WordPress’s ways of saying that a more detailed error, referencing an exact file and line number which caused a specific issue, can be found in the PHP error log file.
Please check for plugin and theme conflicts first by disabling all other plugins and using a default theme to verify the issue is not being caused by an interaction with another plugin or custom code.
If that does not resolve it, please check the error logs for more detailed information about the source of the issue.
Could you please provide your Site Health information to help us determine what might be causing this problem? To get that, you can go to your WordPress dashboard area and go to: Tools > Site Health > Info, then Press the “Copy site info to clipboard” button, and paste the results here.
You could also try installing a plugin like Query Monitor and seeing if it further exposes the error happening on that screen.
Thanks.
Checking the site report led me to discover that Jetpack’s connection to the mothership was broken, and I was unable to reconnect it. I was getting the following errors in the console:
[Error] Failed to load resource: the server responded with a status of 500 () (fields, line 0)
[Error] Failed to load resource: the server responded with a status of 500 () (admin-ajax.php, line 0)
Turning off Jetpack seems to have solved the problem. I’ve been able to set up a relationship successfully.
