The problem might be that your webserver dosen't have write access to the /tmp directory. To solve this you either need to make this dir writable by the webserver or change it to some other directory (PHP-setting).
Its strange that the warning comes from Unknown, and hard to tell whats wrong. Try a reinstall to see if that works. Try to click the finetune button, in the setup wizard, to see if there are some warnings that might cause this.