This issue is can be a result of a couple of issue, either the use of incorrect credentials to authenticate the Connector Server or the JCS service being blocked or stalled.
In the first instance retry the correct login credentials, however if the JCS service is stalled, simply restart the JCS service on the vApp machine by 'restart_jcs' command.
Also check in Windows Connector Server have windows firewall blocking access to necessary ports.
In case can't stop Windows Firewall try add these ports in Windows Firewall
netsh firewall add portopening TCP 20390 "CA-CS SERVER 20390"
netsh firewall add portopening TCP 20410 "CA-CS LDAP 20410"
netsh firewall add portopening TCP 20411 "CA-CS LDAPS 20411"
netsh firewall add portopening TCP 22001 "CA-CS Broker-HTTP 22001"
netsh firewall add portopening TCP 22002 "CA-CS BROKER-HTTPS 22002"
netsh firewall add portopening TCP 20080 "CA-CS WEB-HTTP 20080"
netsh firewall add portopening TCP 20443 "CA-CS WEB-HTTPS 20443"
netsh firewall add portopening TCP 22099 "CA-CS RMI 22099"
The password on the JCS service is set at the time of deployment, if the credentials are not known you may need to reinstall the vAPP.
The same error also can happen after registered and trying to reload to see the registered connector servers.
This can happen if Windows Firewall in connector server or another security program is blocking connector server ports docops suggests disable temporarily windows firewall or if not possible can use this port list to add exception in windows firewall that will allow these ports:
netsh firewall add portopening TCP 20390 "CA-CS SERVER 20390"
netsh firewall add portopening TCP 20410 "CA-CS LDAP 20410"
netsh firewall add portopening TCP 20411 "CA-CS LDAPS 20411"
netsh firewall add portopening TCP 22001 "CA-CS Broker-HTTP 22001"
netsh firewall add portopening TCP 22002 "CA-CS BROKER-HTTPS 22002"
netsh firewall add portopening TCP 20080 "CA-CS WEB-HTTP 20080"
netsh firewall add portopening TCP 20443 "CA-CS WEB-HTTPS 20443"
netsh firewall add portopening TCP 22099 "CA-CS RMI 22099"