Error message after editing a post

When I try to edit a post right after I’ve publiced it, there is no problem. But when I have logged out to the blog, then log in again (or when I’ve written a new post first) and try to edit a post, while publishing I get the following error message:

“You don’t have permission to access /wp-admin/post.php on this server.”

Anyone have any idea what causes this and how to solve it?