Unable to login vCenter through VMWare vSphere Client

[Shakeel Parkar]

error 503- could not connect to vCenter server. unknown connection error occurred.

 

But I am able to get access on individual HOSTS. and Vcenter server is running up. able to ping. 

[Rob Bishop]

can you please give some more details about version of vCenter and whether it is an appliance of installed on a windows server

[Navin A]

It could be because of services being down on the vCenter.
Validate the vCenter version and check on the services status.

[Douglas Bowen]

Because the old c# vsphere client is no longer supported, you need to login to the vCenter via the vCenter IP or full hostname.