I just had this error happen to me, I think for the first time ever. On my C-7, running 2.3.8.134 with all the Z-wave updates installed. I've not changed anything on the hub for weeks; I installed this release as soon as it became available (about 6 days ago).
I've shutdown, unplugged, and restarted and everything is fine again.
This is my "Home" hub, and is enrolled in beta, if you want to pull the logs.
I wonder if this is memory related at all? I can’t prove it, but it seems more likely to happen to me if my hub has less than 190MB of free ram available when the backup runs.
It could be a coincidence, but I’ve now set my C8 hub reboot threshold back to 200MB free ram to test My theory.
I’ve been experiencing this exact same issue. Happened about a month ago and twice this week. Have disabled cloud backups for now until this is fixed.
Send a PM to @bobbyD with your hubs UID (Settings>Hub Details).
That will enable Bobby to look at your hubs engineering logs to help identify why your hub is having this issue.
Just to add this has happened a few times to my c8 w latest s/w. After the error about an unsuccessful backup, I did a successful manual cloud backup and then rebooted in that order
Will do…thanks
It happened again last night, with the hub now running 2.3.8.139.
How much free ram was available when you checked on the hub?
I was getting a couple of failures a week before I changed my reboot threshold to less than 200MB free ram.
I am experiencing the issue on my C8. The issue started about a month ago and the only way I could get the cloud backup to work again was performing a soft reset. I would see the Z-wave radio go down/up at least once per day.
Then last week it start happening again and now it was the Zigbee radio. So I put a support ticket in like 4 days ago suspecting a possible hardware issue. I am wondering if the database is becoming corrupted some how, hence why the soft reset may have fixed the issue. I have not heard back from support though.
This morning when it happen I had 320MB free of memory.
Well there goes that theory, must have been a coincidence that it seemed to help on my C8.
I haven't had this problem for quite awhile, BUT since re-enabling cloud backups, none of my z-wave devices work until they are triggered a second time. On the Z-Wave Details page, all devices are listed but the stats for each device have been erased. Newly triggered devices are then reported at 9.6 kbps with crazy four-hop routes. This improves over the next day until everything is a direct, 100 kbps connection.
Basically, it appears the z-wave radio is partially resetting during a cloud backup.
I don't know, I didn't look ... I've never had this issue twice now.
I had only installed 2.3.8.139 at about noon of the 10th, so the hub was up only about 1d 15h when this happened.
The cloud backup was set to daily so I changed it to once a week and did another soft reset, guess I will see what happens.
What I did notice though is the according to the hub events the Z-Wave radio does not go down at the same time the cloud back fails. Although when the cloud back is successful , the Z-wave radio does not have any issues that day,
I swear there was an update out earlier today that had a possible fix for the cloud backup issue. But when I check now, there is no update?
They must have found a last minute breaking bug and pulled it.
[Release 2.3.8 Available]
.139 was released 4 days ago.
As far as I know that was the last release that I know of anyways
My hub just debunked my theory! 
My hub had even rebooted itself prior to the cloud backup running due to low ram.
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