OK, I'm having the same problem again. The following error comes up when I try to add, edit, or buy an item:
Not Acceptable
An appropriate representation of the requested resource /forums/index.php could not be found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
I'm running SMF 1.1.8, and have nneo's advance shoutbox installed (though I don't think it has anything to do with the error). I'd be willing to make someone admin of the forum if need be.. Thanks for your help!