Having either of these errors can lead to some services not started on the vCenter server.
503 Service Unavailable (Failed to connect to endpoint: [class Vmacore::Http::NamedPipeServiceSpec:00000000011D7F00] _serverNamespace = / _isRedirect = false _pipeName =\\.\pipe\vmware-vpxd-webserver-pipe)
503 Service Unavailable (Failed to connect to endpoint: [class Vmacore::Http::LocalServiceSpec:00000000012B0BA0] _serverNamespace = /vsphere-client _isRedirect = false _port = 9090)
If your vCenter is running on a Windows box, checking the status of its services can provide some first-hand information :
cd C:\Program Files\VMware\vCenter Server\bin./service-control --status
To stop all the services :
./service-control --stop --all
To start all the services :
./service-control --start --all