It was not possible to connect to the redis server(s); to create a disconnected multiplexer

The error you are getting is usually a sign that you have not set abortConnect=false in your connection string. The default value for abortConnect is true, which makes it so that StackExchange.Redis won't reconnect to the server automatically under some conditions. We strongly recommend that you set abortConnect=false in your connection string so that SE.Redis will auto-reconnect in the background if a network blip occurs.

for beginners who dive in other's code an face this problem:

if (RedisConn == null)
        { 
            ConfigurationOptions option = new ConfigurationOptions
            {
                AbortOnConnectFail = false,
                EndPoints = { redisEndpoint }
            };
            RedisConn = ConnectionMultiplexer.Connect(option);
        }

You should also pay attention to the last part of your error message, as it seems to provide very useful details about the reason why connection have failed.

In your case:

It was not possible to connect to the redis server(s); to create a disconnected multiplexer, disable AbortOnConnectFail. UnableToResolvePhysicalConnection on PING

My case:

It was not possible to connect to the redis server(s); to create a disconnected multiplexer, disable AbortOnConnectFail. Timeout