I followed the checklist to configure vCAD when I had the issue. I redeployed it following the VMware documentation Configuring the Cloud Appliances and I was able to access the plugin via vCD now
Here I noticed two things during the redeployment however I am not sure what caused the issue.
1) In new deployment while configuring cloud service I used IP address and port 443 as per the documentation "Enter the VMware Cloud Director URL as https://VMware-Cloud-Director-IP-Address:443/api"
2) In old deployment I believe I missed the tunnel configuration in https://Appliance-IP-Address:8442/ui/admin as it was not part of initial checklist however the system monitoring showed all green.