Well, what is and what should be...

The created and "stored" Wifi-profiles get lost, with next reboot they're just vanished.
It is also not possible to connect to a fresh created profile.
So I gave mo86's method of disabling AP a try.
Now I have no wireless access to the Pi at all, what a pity.
I doubt it will become better when using cable.
Any idea how to store a WiFi-profile which will work and survive reboot?