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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