Today I followed darwindesign’s way, and what can I say: Everything seems to be “tickety boo”.
I disconnected ethernet lan, rebootet and tested wlan only, and all seems to be fine. The wlan connection reads “5 GHz, 866 / 866 Mbit/s, WPA2” on 192.168.1.6, which is ok now.
And I will stick for now with enabled DHCP-Settings in the Fritzbox and my managed HP-ProCurve Gigabit switch, in which I had to re-enable (and replug) DHCP also.