@hoba:
Are you sure that your client isn't causing any backgroundtraffic?
Thank you for this advice.
Wireshark shows some traffic, may be becuase of the external syslog-server or firefox-update, …
It works fine without the patchcord.
Never mind, i have to look for another solution but the idle-timer in windows is just for windows-pcs and an more general solution would be fine.