On opening https://PSC_FQDN/PSC we receive the error: message An error occurred while sending an authentication request to the PSC Single Sign-On server - null description The request sent by the client was syntactically incorrect.
I had the same issue. Took me a while but we finally found the solution ... PSC/SSO certificate expired, and psc-client (which hosts the /psc URL) refused to connect as a result. In our case, this "ssoserver" certificate was replaced in vCenter 5.5 with a CA-signed version, then during the upgrade to vCenter 6, carried over and ultimately expired. Wrote a blog explaining the backstory, findings and resolution. See if this is applicable for you: Troubleshooting Expired PSC Certificates with vSphere 6 | Virtually Understood
Hope this helps!