I randomly have same issue. I am working on one right now is how I came across this thread. only access I have is console and SSH. tried restarted all the service and still get the 503 message and show disconnected in vcenter. VM's are still running and responding.
ESX host responds to pings and can SSH into it fine. Just stops reporting to vcenter and web client is not working. Try to connect back to vcenter and it says host is un-avaliable, as it cant connect to the host on https.
Only way I have fixed this is reboot the host and kill all the VM's. I have not found a fix to this as of yet.
This looks to have been a intermittent issue for a while now and I don't see any fix or workaround for it. not good for a production environment when I have to kill running VM's.
ESXi Embedded Host Client - Bugs
ESXi Embedded Host Client - Bugs
Along with other posts, and only resolution is reboot host, with running VM's.
Mike