Hi! I'm having much the same issue, and I'm using Firefox 13.0.1 in Ubuntu Linux. The error I'm getting is this:
The requested URL /wp-admin/upload.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
I get this when editing media info via the assistant screen and using the categories dropdown to filter categories. If I'm in the list of attachment categories, and I click on the number under the Attachments column, I can get a list of media in that category, though. If I edit via the basic WP media library screen, I can type categories in manually and they save properly.
This is a new install (Tuesday - this is Saturday) and there are a few other plugins:
Awsom News Announcement
BAW Moderator Role
Fast Secure Contact Form
GD Star Rating
Super Tags Widget
WordPress Backup to Dropbox
I think this is going to be exactly what I need to make my life simpler, would love to get it going.