After much swearing, our #Nextcloud server is vertical again!
There's no place like 127.0.0.1...which is interesting, because apparently redis needed it specified on this network, as opposed to accepting 'localhost' like before.
Bewildering, frustrating, and for the moment, now fixed and off my to-do list.
Still need to upgrade the machine, but for the moment, enjoying getting all of our devices talking to each other again.