I’m sure you’re already aware of the excellent Embedded Host Client from VMware Labs, but are you aware that (at the time of writing) their latest version, v6.0, came out on 3 March 2016? If so, then you may have encountered an issue
when updating the fling on ESXi 5.5.

To update to the latest version I usually download the vib file to a datastore and use the following:

esxcli software vib update -v /tmp/esxui-signed-3623722.vib

Unfortunately this time that doesn’t work. It won’t give an error, but it will just skip the vib:

To get around this, simply remove the existing version:

esxcli software vib remove -n esx-ui

Then install (not update) the new one:

esxcli software vib install -v /temp/esxui-signed-3623722.vib

It’ll then install without issue: