If upstream filtering is being done, make sure all FQDN’s and ports are allowed through the firewall for complete NetCloud Manager functionality. See Firewall Ports and Protocols for Cloud Connectivity (requires login) for a listing of the FQDNs and ports used by NetCloud Manager.
Make sure the NetCloud Manager account is not disabled. Attempting to connect with a disabled account will result in an "Invalid Credentials" error.
While rarely seen on networks with an upstream firewall, Upstream SSL Inspection may cause connection issues and result in a "Connection Reset by Peer" message.