'406 Not Acceptable with POST

Today one of the script of web-application stop working giving 406 "Not Acceptable. An appropriate representation of the requested resource could not be found on this server.". There is POST-request sending via AJAX by jQuery to another PHP-script. Nothing was changed in code by our side. What the most interesting, I copy the whole web-app in subfolder test and it works: in site.com it doesn't work, but in site.com/test it works pretty well. App based on shared hosting. Where I should look to solve?



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