An administrator is unable to login to vCenter Server when using the vSphere web client. The administrator successfully accessed vCenter Server using the client earlier the same day. What is the…

The administrator logged in successfully earlier the same day, proving the Flash plugin version was already sufficient and has not changed, so it is not a likely cause of a sudden authentication failure.

Every vSphere Web Client login attempt is authenticated by the vCenter Single Sign-On service; if SSO is stopped or faulted, no user can authenticate regardless of permissions or browser state, making it the highest-priority first check for a sudden login failure.

The web client server may still be serving the login page normally while only the SSO authentication backend has failed; verifying SSO is a more targeted first step than checking the web server process.

Because the administrator authenticated successfully to vCenter Server earlier the same day, their permissions have not changed and are not the most probable root cause of the new login failure.