@tom-carpenter How did you configure the serial gateway in configuration.yaml? can you please post that section?
Also what arduino hardware did you use?
I am having the exact same issue as kobipes. I am also on hass.io and I have not been able to resolve it. I am using an Arduino Nano with the RF24l01 connected to a Pi using USB (ttyUSB0). No go, configuration errors as stated from your post.
However when I use an Arduino Uno on ttyACM i managed to get it working as long as you cold boot the Pi during config changes.
Any help will be really appreciated!