Symptom 1:
Post upgrading SDDCs to version 1.20, Cloud vCenter is not visible in the HLM UI to be managed.
You may see error "One or more vCenter Server systems are incompatible with the vSphere Web client https:/vCenter.sddc-x-xxx-xxx-xxx.xxxxxxxx.com:443/sdk"
Symptom 2:
Initital vCGA setup fails ar Step 4 of selecting admin group with error "Error: Link failed with reason: Internal system error: com.vmware.vim.vmomi.client.exception.InvalidSslCertificateException: Invalid SSL certificate (HTTP 526 status code)"
Validate if there are any expired or weak signature algorithm certificates in On-premise vCenter trusted root store and un-publish them. Please follow steps in the KB2146011 to identify and un-publish expired/invalid certificates and try re-linking the Cloud Gateway.
If it still fails during the setup or you are still unable to see the VMC vCenter, Open a Support Request with VMware Global Support Services and include the below required information.
for i in $(/usr/lib/vmware-vmafd/bin/vecs-cli store list); do echo $i ; /usr/lib/vmware-vmafd/bin/vecs-cli entry list --store $i --text ; done;