On my C5 hub, on the backup/restore screen, it lists 7 recent backups for download. If you try to download the oldest backup it will generate an error 500. I have noticed this behavior through several firmware updates. Screenshots attached.
Can you PM me the hub id this is happening on? I'll take a look at the engineering logs. Can't reproduce the error on my hubs, but logs will have enough details.
When I'm going to do wholesale rule changes I'll download all available backups just in case the most recent one doesn't work.
The other 6 backups are all present on the hub and downloadable. The 7th backup, regardless of it's URL, is always missing. I've been doing the "download all backups available" thing quite often recently.
I was going to just delete the missing backup but thought it might want to be studied to see why. It doesn't affect anything.