Thanks virtualdive, The vCenter hostname and IP address are already in the Trusted Sites in IE, so this is not the problem.
markdjones82, Flash is installed. I never previously modified the security settings, but I have looked at them now by going to the Adobe Flash Settings Manager at https://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager04.html. I have ensured that each item is set to a lenient value, usually "Always Ask." This doesn't seem to have helped.
I notice that if I use Mozilla Firefox, the error message is "The connection was reset. The connection to the server was reset while the page was loading." Chrome just says "This webpage is not available." IE says "This page can't be displayed."
I tried turning off my firewall, but this made no difference.
Thanks in advance for any additional suggestions.