PLEASED DISREGARD, wrong forum. I re-posted this in "New Installs"
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I did 3 fresh installs of 1.2.9 using Softaculous. All 3 have this error. Admin opens OK, but as soon as I click the PLUS in the categories area I get this error. Then the whole admin is down.
Basically, clicking anything in a menu area causes the whole admin to die. Then the frontend goes down too. This is 3 new domains with nothing else on them.
I uploaded the index.php from the posting above and cleared my cache and it did not work. My cPanel shows no server errors.
Not Acceptable
An appropriate representation of the requested resource / 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.