OK, I figured this one out myself.
The symptoms of crashing were not related to the parity rebuild, but coincidental.
The syslog held the key- this several-times repeating error: ntpd[1135]: kernel reports TIME_ERROR: 0x41: Clock Unsynchronized
I found a post on Reddit that suggested turning off ntpd auto update, but that did nothing. However one post there said "check your motherboard battery", so I did that.
Replacing the battery on the motherboard prevented any subsequent UNRAID OS crashes.
Hope this helps someone down the line.
Dennis