I'm dealing with a system where the Quick Search box is not working. Using the browser dev tools shows ajax requests being sent but receiving a '406 Not acceptable' error.
According to https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/406 this relates to content negotiation.
The AJAX URL is:
and the content negotiation headers in the request are:
This matches what I see on another system that is working normally.
This is CiviCRM 5.5.2 on Joomla 3.8.13