update error "Could not copy file.: wp-admin/admin-header.php"

Hi All,
trying to update my multisite WordPress from 3.8.4 to 3.9.2 but keep on getting this message:

Unpacking the update…
Verifying the unpacked files…
Preparing to install the latest version…
Enabling Maintenance mode…
Copying the required files…
Disabling Maintenance mode…
Could not copy file.: wp-admin/admin-header.php
Installation Failed

i also have the “automatic updater disabled”, have any of you had this issue?

any help would be greatly appreciated