This weekend I updated a lot of ESXi servers to the latest version (build #261974) with the Host Update Utility 4.0. This went really smooth, no problems at all. Even bigger was the surprise, when I suddenly couldn't find the 'Hardware Status' tab in vSphere Client anymore.
A quick research on Google pointed me towards the Plugin-Manager (Plug-ins -> Plug-in Manager) and there it was: The vCenter Hardware Status plugin was disabled:
Looking at the error message, it seems that the vSphere Client couldn't correctly fetch the scriptConfig.xml file using port 8443 from the vCenter Server.
Another research pointed me to the information VMware KB 1010641: Enabling the Virtual Server Service status and hardware status plugins fails. This KB information confirmed it. One of the points in the checklist is to verify, that the computer on which vSphere Client is running can connect on port 8443 to the vCenter Server.
To solve it: Check your firewall settings and verify that port 8443 is open. If you can still not connect via a simple 'telnet vcserver 8443' don't blame the firewall yet. On the vCenter server check if the 'VMware VirtualCenter Management Webservices' service is running. It could be that this service is set to a manual startup type so you have to start it manually. Once this service is started, check the connection to port 8443 again. It should work now.