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[Not AcceptableAn appropriate representation of the requested resource /index.php could not be found on this server.Additionally, a 406 Not Acceptable error was encountered while trying to use an ErrorDocument to handle the request.]

Hello I just upgraded to 1.2.9 and my website does not work. It said:
Not Acceptable
An appropriate representation of the requested resource /index.php could not be found on this server.
Additionally, a 406 Not Acceptable error was encountered while trying to use an ErrorDocument to handle the request.
What can I do, Please help.
Thank you.