Every time you get an error that tells you it cant open a file, you have 3 reasons,
1. Permissions are wrong, if they are wrong, the files will not be read and the server markes them it cant open them
2. Files are missing, the server cant find the files because they are not there where it searches for them.
3. The paths are wrong, the server looks for a file according to what the DB tells it to, but the server cant find the file/folder on that location
Download the repair settings.php fron the SMF forum, upload it to the folder you have your forum installed to. point the browser to the file and make sure your forum have the correct paths there, AND that your forum are connection to the CORRECT database.