When attempting to connect to a vCenter Server, an administrator observes the following at the top of the vSphere Web Client: Could not connect to one or more vCenter Server Systems:https://vCenter.co

If the vCenter Server host is powered off, crashed, or blocked at the network layer, no TCP connection to port 443 can be established, producing the SDK endpoint error shown.

The vSphere Web Client authenticates through SSO before accessing the vCenter SDK; a stale or incorrect SSO service registration for the vCenter instance prevents session establishment and results in the connection failure.

TLS certificate validation requires the Common Name or Subject Alternative Name on the certificate to match the FQDN in the URL (vCenter.corp.com); a mismatch causes the Web Client to reject the connection before the SDK handshake completes.

An external PSC is a fully supported and standard deployment model for vCenter Server; its presence does not interfere with Web Client connectivity to the vCenter SDK endpoint.

An incorrect DNS entry would prevent the client from resolving the FQDN at all and would produce a name resolution error rather than the specific SDK endpoint connection failure error shown, which implies the hostname resolved successfully.