I had a VMware ESXi 4.1 host that I was unable to update/patch using Update Manager the today.
It was coming up with the following errors when I tried:
VMware vCenter Update Manager had an unknown error. Check the Tasks and Events tab and log files for details.
And when I did that, I found this:
Could not install patches on esxihostname
Remediation did not succeed for esxihostname: SingleHostRemediate: Install error on host: esxihostname, error details: vim.fault.NoHost.
So I did some quick googling, and ran across this website that had the answer I was looking for.
The particular host that I was trying to update did indeed have an Unknown (inaccessible) VM on it (left over from yanking the storage and being sloppy...). I removed the VM from inventory, then was able to successfully patch the host.