I installed a clean v1.3.1 twice with the same problems on a LAMP server.
Safari Mac - works
Firefox Mac - problem (cleared cache, cookies, etc)
Firefox Windows - problem (cleared cache, cookies, etc)
I am getting a similar problem as other posts on here. I am able to see the login screen but as soon as I login via Firefox I get:
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Not Acceptable
An appropriate representation of the requested resource /opengoo/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.
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I have a separate install of Opengoo 1.3 on the same machine and I have never experienced this problem with that version.