the screenshot was made using ISO 1600 and 15sec, but I also tried ISO 6400 and up to 60secs (would have entered longer exposure time, but 60sec seems to be the limit - makes some sense for unguided imaging).
(Yes, nice idea: perhaps the ISO not used internally for some reasons. But changing it here also changes the value in the capture module.)
Did another research during daylight. I captured an image with different exposures - without any stars of course - and could see some images. But I also got a message that the image could not be downloaded (unspecific error) - this happened very often. I also tested using GPhoto driver. I will retry it in the next cloudless night using GPhoto, because this Is just something I did not try yet.