EDIT:
Problem resolved.
I waited several days, tried a reinstall, and it came in clean this time. Also, I had another install on another Unraid Box which gave me verification of the path settings that I used previously (yay for redundancy).
Reinstall, duplicate settings, bazinga, back up and running. Did a test restore to verify, and was able to get the file back from my offsite backup. Looks like it originally died February 14th, which coincides with me doing a large file transfer, which filled up Cache and robbed storage from my VMs and Dockers (also on cache drive) and caused all sorts of mayhem.
Original message below: