I don't think this is something to do with kernel or grsecurity patch. It is mostly related to mod_security by guessing the error 406
Just some guessing
Giam Teck Choon
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Yes the 406 error should be related to mod_security. You can turn it off in httpd.conf under SecFilterEngine. Stop and restart httpd. Thanks to Jonathan Michaelson of WayToTheWeb for the original information on this.