Wireshark would not allow me tick the box for “Monitor mode” (it auto-disabled itself again), and “sudo iwconfig wlan0 mode Monitor” says “device busy”. However, using airmon-ng to enable monitor mode worked: sudo airmon-ng start wlan0 {channelnum} (the channel number might be optional) gives a second mon0 interface which allows capturing in monitor mode just fine – at least if I disabled the use of encryption for the wireless network. I didn’t manage to convince Wireshark to decyrypt WPA2.