We were migrating 40 VMs from one Netapp filer to another and implementing IPSpaces at the same time, so we needed to power down all 40 VMs for this.
Once the VMs were migrated, we powered on the DB VMs first - no issues.
Once the DBA verified Oracle was up, I started powering on app VMs.
But they all hung on
"unregistering virtual machine from source host system"
Turns out this was due to 3 of the vmwaretools updates hanging - I had started these vmwaretools updates while the DBA was working on Oracle.
These were not the same App VMs I was trying to power on, but once 2 of the hanging vmwaretools updates finally timed out, some of the App VMs finished powering on.
I manually killed the remaining hung vmwaretools update using the ESXi command line then everything proceeded fine.
Hope this helps someone else since it was not obvious at the time the vmwaretools updates were preventing other VMs from powering up.