So this problem is very strange because for me doesn\'t work Wordpress media library in Wordpress admin menu only grid mode, its very strange problem because this problem oc
I had exactly the same error. I could not search for plugins, get the list of my uploads, everywhere where an ajax function was triggered, I needed to reload the page to see the results.
clean your functions.php and see if it works now. if yes, clean up your functions.php search for spaces or not set closing tags.
If the problem is with the user and seeing that you have tried almost all, you can try deleting the user and creating it again.
this type problem is create by functions.php file , remove the code of you added in functions.php and than see it work proper.
standerd code write in functions.php file
Got exactly the same problem and some ajax error on revolution slider too, and after searching for long time i resolved my problem.
It was just an HTML comment in functions.php on child theme, after remove it, everything work perfectly for me.
Same problem with space in function.php don't cut your code with space between php tag
In my case, there was an error in the backend. You should check your network tab in developer tools and find admin-ajax.php. Even if it returns HTTP 200, it shows an error in the response body.
From the information provided, you can work on which plugin, theme, or whatever file causes the error.