I think I see the problem.
When you click "preview post" - it should open the post in a new window or tab and your edit post page should remain open in it's own window/tab. If you're on the preview page, you should not click "edit" to make more changes but instead you should go back to the original edit post window or tab to make your edits. Making changes in the post editor and clicking preview again will refresh your original preview window to show your latest changes.
Here's a screenshot that shows the two different windows that are opened when clicking "preview" in the post editor: http://d.pr/i/HvqM
We've recently made changes to how posts are edited in a multi-author environment, so that might explain why the workflow you used to do doesn't work anymore. Here's more information on those changes: http://en.blog.wordpress.com/2013/04/11/autosave-menus-locking/
Try following the instructions I gave above to edit and preview your post and let me know if you still see the error.