Connect to hidden Wifi AP with wpa-supplicant?

Finally, I've found out a solution, just an other line (previous not needed: wpa-ap-scan)

wpa-scan-ssid 1

I've not really found it in any documentation... just in a forum post.

I've not found official documentation about the /etc/network/interfaces config.

You can find the documentation at /usr/share/doc/wpasupplicant/README.Debian.gz. Under Chapter 4: Troubleshooting there's a section titled Hidden ssids:

In order to be able to associate to hidden ssids, please try to set the option 'ap_scan=1' in the global section, and 'scan_ssid=1' in your network block section of your wpa_supplicant.conf file. If you are using the managed mode, you can do so by these stanzas:

iface eth1 inet dhcp
    wpa-ap-scan 1
    wpa-scan-ssid 1
    # ... additional options for your setup