So, here are new details:
Almond IP 192.168.1.1
Subnet 255.255.0.0
If I turn WLAN 2,4 GHz on, first external Connection (to WAN) gets lost, then no more connection to router.
If I turn off WLAN 2,4 via LCD, connection comes back.
Tried with WPA2 and no enc, both show same error.
No such issues by now with 5 Ghz.