In my case that was an issue related to the date / time. The SSO service was not starting and make 400 / 503 errors on the web interfaces.(vcenter 503 Service Unavailable (Failed to connect to...
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In my case that was an issue related to the date / time. The SSO service was not starting and make 400 / 503 errors on the web interfaces.(vcenter 503 Service Unavailable (Failed to connect to endpoint)... and [400] An error occurred while processing the authentication response from the vCenter Single Sign-On server) I had to correct the time zone int the admin interface (https on port 5480) and reboot.
