The web client does not load. Accessing https://example.local/ui/login eventually gives this error:
A server error occurred.  An error occurred while sending an authentication request to the vCenter Single Sign-On server - An error occurred when processing the metadata during vCenter Single Sign-On setup - java.net.SocketTimeoutException: Read timed out. Check the vSphere Client server logs for details.
This is not always a problem. While it could happen when the system hangs, it also naturally happens for about a minute (depending on server speed) after restarting the service, which means it is working.
The vSphere Client web server is initializing The vSphere Client web server is still initializing. Please try again shortly.
Connect to the vCenter VM via SSH or by clicking the browser console within ESXi. Restart the web client.
service-control --stop vsphere-client service-control --start vsphere-client # or service vsphere-client restart
Reset the vCenter VM entirely. It will not impact the other instances on the ESXi server.