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 most

While the vSphere Web Client did use Adobe Flash, a Flash version issue would be a persistent problem across sessions and would not suddenly cause a failure that worked earlier the same day.

vCenter Single Sign-On is the authentication backbone for all vSphere Web Client logins; if the SSO service stops running, no user - regardless of permissions - can authenticate to vCenter. Because the administrator successfully logged in earlier the same day, permission or browser issues are unlikely, making a service disruption the most probable cause to check first.

There is no distinct 'vSphere Web Server' service separate from the SSO and vCenter services in typical deployments; verifying SSO is a more precise and impactful first step for authentication failures.

Permissions are unlikely to have changed since the administrator successfully logged in earlier the same day, making this a lower-priority troubleshooting step than verifying core authentication services.