At this point, we’re grasping at straws.
I know how frustrating things like this can be.
Did you change the files to 644? Especially the manager/includes/config.inc.php and index.php files.
You might ask the host to change the error reporting level to see if that gives you anything.
You can change it yourself in /index.php, /manager/index.php, and manager/includes/config.inc.php
Change it from:
error_reporting(E_ALL & ~E_NOTICE);
to
(Don’t forget to change it back later.)
As a last resort, I would do an upgrade install. It might fix things, or give you a helpful error message.