@RAlm: The “obvious” suggestion was the one that fixed it. Redis 6, as you may know, changed security a bit. To my knowledge, I never entered a password in redis.conf, and I didn’t have this problem previously; perhaps Redis 6 changed that behavior.
Either way, I added this line to the redis.conf file, and the error is gone:
requirepass <PASSWORD_FROM_NEXCLOUD_REDIS_CONFIG_FILE>
It seems there’s a better solution, but I didn’t have time to learn about Redis ACL, which is I think where I need to look next for a long term fix.
Thank you for your help!
buzz