Hi,
I see the issue now. Something is changing the ajax response, adding HTML tags to it, that should not be there. See example response: https://i.imgur.com/Hbv3rnz.png
The body, html tags should not be there. This is very hard to tell what exactly might be causing this. It can be a plugin, as well as a cache or CDN or a server module, that uses some sort of HTML correction.
The only way to resolve this, is by finding what is adding the extra data, and disabling it. I would suggest first turning off the plugins one-by-one to find which might be conflicting. If none, then looking at server cache, then CDN services (HTML minification), and adding the ajax handler files (admin-ajax.php and ajax_search.php) to the exceptions list, if possible.