Make sure that there is network path from the Application Server to the target and back, that the RSCD Agent is installed and running and that the ACL's are set to allow access.
1) No such device found
The device is not found on the network
2) Connection refused
4) Connection timeout
There is a connection problem - the computer may be down, or te agent may not be running
3) Login not allowed for user
There is an access permission issue. Check the Agent ACL