OK - here is the anwer:

it came from Air Avionics and states that only one device at a time can be connected via WiFi. That's it...

Does anyone have practical experience with an Air Avionics AT-1 and SkyDemon ?

The point is, that the pilot and the co both like to use SkyDemon on their own devices at the same time during flight and in an individual way. Until now we were using a TRX-1500 with a self constructed WiFi Interface which - in contrast to the original Butterfly Connect - is able to provide position and traffic data to up to 4 WiFi (Android in this case) decives at the same time.

Unfortunately the TRX GPS receiver has died shortly and I am lookiing for a new device. TRX1500 is not available any more so the AT-1 seems to be a good upgrade, especially because it has already integrated WiFi and supports a bunch of protocols on all of the three serial ports and on the WiFi too (that's what the manual says...).

In the web I found several comments about instable WIFI connections, but they were all a bit older - maybe the problem is solved in between ?

- Thanks for your comments ;-)