Same here, ESXi host 6.5 happened after I installed updates on the server so it was in maintenance mode.
The httpd service was stopped so I started it, but still no change on the host/ui
I rebooted the host and now the ui loads.
On this link - it seems there is a bug with the esxui,
I checked from SSH
/etc/init.d/esxui status
it was running, so possibly /etc/init.d/esxui start may help if it is stopped. Others reinstalled the VIB.
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