it was difficult to track down the problem, as it showed up very sporadically. Actually it looks like it is related to a weak wlan connection. How this can cause a crash I dont know but since I changed the antenna, the problem disappeared. It looks like that -at least in my case- the built in ceramic antenna is rather a dummy load than an efficient antenna. Fortunately it is possible to connect a external antenna, which in my case is a 31 mm whip at the other end of a short coax-cable tail, while the 31 mm braid is pulled back over the coax-cable. This mod also requires the 0 Ohm resistor-jumper to be resoldered to the antenna socket side - which is quite tricky btw.