Redis cache server not running. Please contact Administrator / Tech support on bench serve --port 8001

When i run “bench serve --port 8001” the server is started and im able to login too, but im getting an exception “Redis cache server not running. Please contact Administrator / Tech support”.
But on “bench start” which by default start the server on port 8000, everything is working fine, im not getting any redis-server exception.
can anyone explain why its happening?