It seems the connection to Redis is closed when deploying a flow, and not re-opened afterwards. Deploying any change requires a full restart of Node-RED to restore the connection:
Aug 23 11:40:13 machine Node-RED[22415]: Unhandled rejection Error: Connection is closed.
Aug 23 11:40:13 machine Node-RED[22415]: at Redis.sendCommand (/home/nodered/.node-red/node_modules/ioredis/lib/redis.js:558:20)
Aug 23 11:40:13 machine Node-RED[22415]: at Redis.rpush (/home/nodered/.node-red/node_modules/ioredis/lib/commander.js:131:17)
Aug 23 11:40:13 machine Node-RED[22415]: at RedisOut.<anonymous> (/home/nodered/.node-red/node_modules/node-red-contrib-redis/redis.js:166:37)
Aug 23 11:40:13 machine Node-RED[22415]: at emitOne (events.js:115:13)
Aug 23 11:40:13 machine Node-RED[22415]: at RedisOut.emit (events.js:210:7)
It seems the connection to Redis is closed when deploying a flow, and not re-opened afterwards. Deploying any change requires a full restart of Node-RED to restore the connection: