Arduino Yun & nRF24L01+ -> Linino crashing
I finally managed to get it running. It really seems to be a problem with radio interference. I failed to hook up an external WLAN antenna to the Arduino Yun mini but I found a very useful workaround with a usb adapter here:
http://www.lucadentella.it/en/2014/11/08/yun-adattatore-wifi-usb/

This workaround is possible with an Arduino Yun mini as well but you'll need the Arduino Yun Linux OS from here:
https://www.arduino.cc/en/Main/Software

because the necessary packages are not available for the Linino system that comes with the Yun mini. Then, you can connect the WiFi adapter to USB+, USB-, 5V and GND of the Yun mini and you're good to go.