I've searched the forums for insight on this error message:
You do not have permission to preview drafts.
On some WordPress installations, this message appears when trying to preview revisions of a published post. I consider this a bug since anyone authorized to edit a post should certainly be authorized to preview the result. This error occurs even when the user is an administrator, which means the error is obviously a mistake.
Some forum posts suggest that the solution to this problem is to go into General Settings and edit the WordPress Address (URL) and Site Address (URL) so that they match. That's just a workaround, not a true solution.
There are legitimate reasons for these two URLs to be different. For example, this makes it possible to use a shared SSL certificate on a shared web server to secure the site login and back end administration while displaying the front end on a custom domain name. I do this all the time to improve site security without incurring the extra cost of a dedicated SSL certificate, dedicated IP address, etc.
In summary, preview functionality should not be lost simply because the two URLs don't match! Can we please acknowledge that this is a bug, not a feature, and get this fixed?