TL-DR - does your Health Check SR still exist? It turns out mine didn't!
This a story about finding an unhandled edge case in the Xen Orchestra Backup [NG...? it's just the backup tool now and we don't call it "NG" anymore, right?] utility: When you delete the SR to which your backup job restores vms for Health Checks, it fails without much helpful information.
On the latest commit to master:

So I recently moved my vm disks to a new SR, made the new SR the pool default, and removed the previous SR. Then I noticed that my backups were failing and I was getting no error message. It was quite strange. I decided to update my Xen Orchestra "community edition" (installed using the ronivay XenOrchestraInstallerUpdater tool) to the latest master commit, but the issue was still happening.

An example log from this evening before I solved the mystery:
{
"data": {
"mode": "delta",
"reportWhen": "failure"
},
"id": "1740978958075",
"jobId": "9017a533-4a2a-42ad-9319-cba19247e062",
"jobName": "Daily Delta Backup of step-ca at 7:05pm",
"message": "backup",
"scheduleId": "a2229c74-dc47-42f6-90fd-a86ef7e6529d",
"start": 1740978958075,
"status": "failure",
"end": 1740978959190,
"result": {}
}
And when I went to the log entry under the Settings menu in Xen Orchestra, I saw an empty error message and this text when I clicked the eyeball icon to display details:


And it wasn't just one backup job either, over the course of the next day 3/4 backup jobs that all point to different shares on the same backup host were all failing -- but I hadn't made any changes to the shares or the underlying storage on that host so I really wasn't sure what could have caused it. Anyway it was the end of the weekend and time to go to bed.
This is all in my homelab so it's not a big deal if i miss backups for a few days, I was doing this on the weekend near the end of February and I knew I was a few days before an update which is probably when a number of last-minute approved commits get merged, so i figured I would wait a few days for the dust to settle and it would sort itself out after I update again once the next official release at the end of the month.
Just tonight I decided to update to the latest Xen Orchestra again, and my jobs still failed, like immediately with no error message. I did a bit of googling and found One of the backups fail with no error.
After skimming through I noticed their reported results were really similar to mine, but I hadn't restored from a backup and I didn't know what was wrong. I figured it would be just as easy for me to follow the same advice given, however: to recreate the job.
As I was referencing the schedule of the original job I noticed that the Health Check SR was in red text and just showed an unknown uuid which is when I realized that my backup jobs were still configured to restore Health Check VMs to the SR that I had destroyed and I had forgotten to update my jobs to restore to the new SR.

So, I am partially sharing a learning experience, and also report that the error handling for this situation ought to be improved.
I have replicated this many times in my instance and I would be happy to provide any logs that might be useful beyond what I've already included.
Anyway, thanks for making this awesome project open-source so people like me can tinker at home.