I experienced this issue in my lab this week and it turned out to be that the vmware-psc-client service had not started on my external PSC. I realized this by first checking the logs on the VCSA:
SSH to the VCSA and run cat /var/log/vmware/vpxd
Notice how there is some error connecting to SSO? Try to connect to SSO VMOMI endpoint
Since I am using an external PSC, I then SSHd into that and checked the services with service-control –status –all
As you can see above, the vmware-psc-client was not running so I ran the following to start it: service-control –start vmware-psc-client
The service is now started and I was able to refresh the VCSA Web Client and login
VMware have a full KB on the 503 errors but my issue was not listed: https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2121043
