What is happening?

The HyperText Transfer Protocol (HTTP) 406 Not Acceptable client error response code indicates that the server cannot produce a response matching the list of acceptable values defined in the request's proactive content negotiation headers, and that the server is unwilling to supply a default representation.

What I can do?

If your are a website visitor:
Make sure your browser is up-to-date and your HTTP request is correct.

If you are the developer or website owner:
Make sure your HTTP request is not subject to a block rule on ModSecurity rules enabled on your hosting account.