Hm... I've changed the settings to "everything auto" (enabling wireless-b
and all that stuff...) -- but now it simply worked:
A "iwlist wlan0 scan" reports them with:
network 1 [not working before, AP FritzBox 6369]
Quality=50/70 Signal level=-60 dBm
network 2 [working before, D-Link DIR-850L]
Quality=54/70 Signal level=-56 dBm