Nextcloud: RedisException: Redis went away

What the message looks like

{"reqId":"Yk3x...","level":3,"message":"RedisException: Redis went away in /var/www/nextcloud/lib/private/RedisFactory.php"}

Common causes

Redis is not running: it crashed, was stopped by an update, or was killed by the out-of-memory killer.
The redis block in config.php points to the wrong host, port or socket path.
Redis listens on a Unix socket, but the web server user is not in the redis group or unixsocketperm is too strict.
Redis closes idle or overloaded connections, for example because of a low timeout, maxclients or a memory limit.

Check

Is Redis running?
systemctl status redis-server --no-pager
Does Redis answer on the socket?
redis-cli -s /run/redis/redis-server.sock ping
What does Nextcloud have configured?
sudo -u www-data php occ config:system:get redis
Why did Redis stop?
journalctl -u redis-server --since '1 hour ago' --no-pager | tail -n 20

Fix

Start it and keep it enabled: systemctl enable --now redis-server.
Make Nextcloud use the same endpoint Redis listens on, for example occ config:system:set redis host --value=/run/redis/redis-server.sock and occ config:system:set redis port --value=0 --type=integer.
For a socket, add the web server user to the group: usermod -aG redis www-data, set unixsocketperm 770 in redis.conf, then restart Redis and PHP-FPM.
If the kernel log shows Redis killed for memory, give the host more RAM or lower the other services. Set a maxmemory that fits.
Check nextcloud.log again after a few minutes to confirm the error is gone.

FAQ

Can I just remove Redis from config.php?

Yes, as a test or a stopgap. Nextcloud then falls back to the database for file locking, which is slower and can cause more lock conflicts. Fix Redis rather than leaving it out.

Is the data lost when Redis goes away?

No. Redis holds locks and cache, not your files. After a restart the locks are rebuilt. Stuck locks from a crash can be cleaned with the file-locked entry in this lexicon.