Page Returns 403 error

403 (forbidden) happens when the process tries to access some resource it does not own permissions to get. Since the post saved, its probably not wp-content. Suggest you check this server’s error logs to see if they will tell you which resource it was trying to write to. You may need to change some permissions.

as @mechx1 says, you will need to change the permissions of that file so that the server can access it. Here’s a WP Codex entry on changing permissions – scroll to using FTP client.