How did you try to download ?
Did you test it in the connection tester from the exact poller .?
Is this for NCM (Network Configuration Manager) or SCM (Server Configuration Monitor)?
Are you trying to download a network config from a switch, router or firewall, or a configuration of a server or application?
Issue has been resolved. Issue was with the polling engine. When I did migration and upgrade polling engine was removed from the list. So I manually add the polling engine to all the nodes.