Huh, interesting… is that the same error you had before?
Maybe there’s still some modification to system files left somehow. I wouldn’t now how, but who knows 
OK, here’s a crazy idea:
- Install the Magisk app
- Boot (not flash) your prerooted boot.img
- Perform a Magisk uninstall in the Magisk app
That shouldn’t work, a factory reset should have purged all the remnants of Magisk off of /data, but hey, worth a shot.
Apart from that, could Titanium Backup have messed this up somehow, I don’t really know how it works these days, haven’t used it in almost a decade…
Third option, your other boot slot could be tainted, maybe because you flashed a wrong boot.img to it at some point. The updater usually flashes to the unused slot, maybe then, the failure gets triggered.
So try flashing that boot_a.img you used to get your phone working again to the other slot.