Hey,
I had the same issue yesterday (but my end-device reported it over serial). It worked before and after tinkering with the application key I had this issue. I solved it by deleting my device and re-adding it. Make sure that your application key is the right way around (MSB/LSB) or try adding two devices where you try both ways (one where you paste your AppKey as MSB and one where you paste your AppKey as LSB).
I hope this helps.
Greetings
Mika