[Update #4 A full two days later] Finally everything is back to normal. I believe the issue came down to using the built in device handler for my Danfoss Living Connect radiator valves, all of them were in error state I noticed, so I changed to a custom device handler I had used in our old home, and although it is not local execution no more errors, strangely enough my Z-wave network became stable about an hour later, I had to run a repair about 5 times, but all seems good and even better than before. I also changed all the device handlers for my Fibaro plugs to the built in one, and I don’t know if that helped or not. I almost completely reset my hub and was about to start from scratch thinking something had botched up the Z-wave config.
On another weird note, a majority of my webCore pistons stopped responding to their triggers, I had to duplicate the pistons and pause the old ones and everything was fine with them after that?
[Update #3 24 hours later] - Complete frustration, so many z-wave devices offline and only randomly working from time to time, I don’t know if I have a hub failure, or a bad device causing havoc. I’ve cut the power to nearly all devices. Working from a circle closest to the hub outward, but even then I was only able to join one device and now it won’t join other devices (battery powered door sensor).
I’m really at a loss for any ideas to fix this. Still not a single word from my support request.
[Update #2 8 hours later] - it’s happening again, although not with all Z-wave devices, but certainly all plugs, this is incredibly irritating and I can’t think of any changes that it might be attributed to, back to the troubleshooting.
[Update 3 hours later] - everything is magically working again… able to re-include the reset devices w/o issues as well - must’ve been server-side 
patience is not my thing.
This morning (Europe) I began noticing odd behavior, and realized none of my z-wave metering plugs, nor metering switches (different brands) were working properly, or at all. FYI I’m still on Firmware 31.00004, no update yet…
In order to troubleshoot I reset one plug, and tried to add it back, my Smartthings V2 hub would would not include it. I did this on another one and same thing, finally did this on a door/window sensor and it also would not include.
Needless to say this has left a number of big gaps in the network (since I basically have no functioning repeaters now) and much of my setup is not working. I am seeing a lack of Z-wave device activity in my logs, a few multisensors (battery and powered) and door/window sensors are working.
I’ve tried MULTIPLE z-wave repairs, no errors except for a few phantom devices, which I have tried to remove the old app just attempts a force-remove (if I create a dummy device with the network ID of the reported device) but doesn’t stop and it’s not removed from the IDE, the new app claims the device is working properly and can’t be removed. I don’t think this is related to the issues, as these have been around for a few months.
I have also power cycled a number of times, letting the hub stay off (w/o batteries) for over 10 minutes, etc.
Any ideas, thoughts? I’ve already opened a ticket with support, but I don’t want to wait for that black hole to finally open up.