I googled the Error message, and it seems like it can be caused by a a multitude of different things. Maybe it is easier if you uninstall Redis completely and start over with a fresh install:
Remove Redis completely, including the config file:
apt purge redis-server
rm -rf /etc/redis
reboot
Reinstall Redis:
apt update && apt full-upgrade
apt install redis-server
Re-configure Redis:
cp /etc/redis/redis.conf /etc/redis/redis.conf.bak
sed -i "s/port 6379/port 0/" /etc/redis/redis.conf
sed -i s/\#\ unixsocket/\unixsocket/g /etc/redis/redis.conf
sed -i "s/unixsocketperm 700/unixsocketperm 770/" /etc/redis/redis.conf
In the following command you have to replace <secret> with the password from Nextcloud’s config.php:
sed -i "s/# requirepass foobared/requirepass <secret>/" /etc/redis/redis.conf
sed -i "s/# maxclients 10000/maxclients 10240/" /etc/redis/redis.conf
usermod -aG redis www-data
cp /etc/sysctl.conf /etc/sysctl.conf.bak
sed -i '$avm.overcommit_memory = 1' /etc/sysctl.conf
sysctl -p
systemctl restart redis-server
Hope that helps.