It looks like your serial ports are not translating properly to actual /dev/tty devices all the time. This looks to me like the device is dropping off the usb bus. Sometimes the translate gets a result and other times not. The only reason that would happen is if the device is not present anymore. You might turn on LV_STATUS and LV_DEBUG loglevels and tail the HAL log to see if the device is disconnecting.
I got tired of trying to find where the issue was and migrated to 14.04, and all problems gone!. Furthermore, I replaced the z-stick s2 with a gen5 and Incredible positive results!. Fast response zwave network, very stable so far with 19 devices. Im not even running the stick on a dedicated usb powered switch!! No retries nor failed packages on any of the devices. I hope this helps others.