@Stivi, you forgot to mention that you have your Electron plugged into an Asset Tracker Shield 
This does change things quite a bit and also counts as leading us on 
Asset Tracker Shield already uses A2 as SPI SS for the accelerometer
https://github.com/spark/shields/blob/master/electron-shields/asset-tracker/pdfs/asset-tracker-sch.pdf
You need to select a free pin and assign that as new SS for the RFID reader in your code.