These days I had a strange scenario in a large customer environment. Every time I tried to start a workflow via SOAP from a webservice client there was an error. After regarding this in the vCO client I found out that every run from the external webservice client causes the action "getAllVMs" to fail. Disconcertingly the array "VMs" wasn´t filled by the action and there was no error message.
The error occurs later in the workflow because of the empty array. What the problem was? I described it here: http://mighty-virtualization.blogspot.com/2011/03/vco-internal-methods-and-authorizations.html
I hope this helps you to avoid the same problems :-)