Do you have any add ons?

Sounds like some bad or missing files on the server ...

If this is a clean install, you may want to try loading all the files for /admin to /your_secret_admin_dir and see what file(s) might be bad, missing or corrupt ...

You could just up load the one file:
/admin/includes/functions/general.php

but you need to wonder if other files might also be damaged or corrupt ...