DXmanager communicates with an agent on each host called DXadmind. While the DXmanager server is running, it maintains monitoring connections with each agent to gather statistics and status information of each DSA instance.
From time to time these links are dropped (DXadmind restarts/network outages) and need to be re-established. There is a pool of connections used for monitoring, the above error occurs when this pool of connections is exhausted, therefore, a connection cannot be made to DXadmind.
In the short term, stopping and starting DXmanager should refresh the connection pool.
Earlier versions of DXmanager leaked connections, that is, if a connection attempt failed then it could not be re-used. This has been fixed in later versions of DXmanager (from r12.0 SP12 onward).