It crashes again after upgrading the mobo firmware. I'm officially out of ideas.
Interestingly, when it has crashed during these parity checks, according to the hourly array health notifications, it seems to crash in 4-5 hours, but then I'll log in the next day to find that the system has restarted and has had uptime for 12+ hours (with the array offline).
I'll note that the last few parity checks were in maintenance mode, so it's unlikely to be a filesystem-related issue.
I think this evening, I'll run a live Debian environment on this system instead of Unraid, so that I can make sure all of my disks are backed up to tape. I'll report back on system stability when it's just running Debian, because if it's still crashing, then presumably, it's hardware. If it's not crashing, then we can assume that it's Unraid being incompatible with this particular hardware for some reason.