Hi Thanks very much for your response. sudo service pihole-FTL restart was successful.
My guess is that the timing does match, as you say. We don't know exactly when it failed as on our network we have access to two PiHoles -- one on the local L2 segment and one at a different location -- L3 and accessible via "nailed up" VPN. So, although the primary failed, the secondary kept working. So, the effect was just slower DNS resolution -- but not dramatically slower such that we'd necessarily notice it.
I'll turn off DNSSEC and see if that makes a difference vis-a-vis stability. And, I'll try to follow your suggestions if it crashes again.
TU!