You are getting a 403 error on the ajax_search.php file – it means that the installation is forbidden to access the custom ajax handler.
On the Compatibility Settings submenu turn off the Use the custom ajax handler? option: https://i.imgur.com/Zykr9ag.png
After saving, make sure to empty the cache and refresh a few times to make sure this change is applied. It should solve the problem.Best,
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