New Install deactivated visual editor and SEO fields do not show up Installation / Upgrading - All in One SEO Pack Forum

I've just set up a brand new site with the fresh Wordpress install and the latest All in One SEO pack plugin. When I activate the plugin, I can configure the plugin settings but when I try to edit an existing page or post, the visual editor is disabled (the editor toolbar disappears and the content just shows HTML code) and the All in One SEO fields do not appear on the page to edit. I can create a new post or page and the visual editor is still disabled but the All in One SEO fields do show up properly.

I've uninstalled and reinstalled the plugin and have tried older versions. And what really puzzles me is that I have half a dozen other Wordpress sites running the same current version of Wordpress and All in One SEO without any problem.

Does anyone have any idea why this isn't working or where I should even start looking for the problem?

Thanks
Pete