After setup the nice astroberry image and fine tuning of the system i recognised temporarily WLAN loss in a regular base.
Looking in the log files I got no explanations for this effect.
Searching in the web i found some information that the COUNTRY=PL setting could be the problem.
So I changed the COUNTRY language setting to DE which was set also in mein raspi-config settings.
After this was done the nasty temporarily WLAN loss incidences stopped.