Are you accessing the vCenter server or ESXi host.
Is your host and vCenter are healthy and accessible via webclient ?
Have you tried to accessing server from other machine using vsphere client ?
similar issue was found in other link, check if this helps
What's your OS version? .NET version? Looks like SSL/TLS issue. Did you have any disabled TLS version in the local OS registry or in your host using the vmware TLS configuration utility (1 , 1.1 etc)?