How to fix 406 error?

Question

Debugging

Today, submitting a form in a web application stopped working - error 406. When submitting via AJAX, a request is made to another PHP script (using jQuery), but now a 406 error is returned. Nothing has been changed on the site for a month.
At the same time, if you copy the entire application to a separate directory, everything works. The application is on a virtual hosting. What can be seen to fix it?