I see this one a fair bit, both in my own lab and in production environments, when doing vCenter updates.
The simplistic solution is to ssh to the vCenter appliance, log on with the root crednetials (not the Single Sign On ones), open a bash shell by typing shell, then removing a conf file:
rm /etc/applmgmt/appliance/software_update_state.conf
Retry the update from the VAMI interface again.
Anecdotally, it might coincide with aborted upgrades, but currently unable to confirm.
