That error--which I missed when mentioned above--could be Z-Wave database corruption. (The device still being responsive is usually code 2.) A recent firmware update made this less likely to happen but won't do anything to fix existing corruption. I'm not sure there's any way besides resetting the radio and rebuilding the network, though I'd probably just try PC Controller or another secondary to see if that works--probably easier. 
(But yeah, it does require some extra hardware. The newer Z-Wave database should be more resilient to this issue--FWIW, I redid my entire network on 2.3.0 recently and was shocked at how much smoother it went than the early days.)
This post/thread has more on that "0 SDK failure" thing:
