406 errors are typically caused by mod_security. Please see the following links on this.
<a href="http://www.inmotionhosting.com/support/website/website-troubleshooting/406-not-acceptable-error-message" target="_blank">406 Not Acceptable Error Message</a>
<a href="http://www.inmotionhosting.com/support/website/website-troubleshooting/what-is-modsecurity-and-why-is-it-important" target="_blank">What is Mod_Security and why is it important</a>
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