In the inventory tab you should have both a vCAC Infrastructure Administration tree and a vCloud Automation Center tree. If you don't you may be missing a plugin somewhere. Do you have the vCenter plugin configured for your vCenter server? Could you post the workflow you are trying to use here?
I can't think of anything else to try. The issue seems to be that the action to get the VM object through the is not retrieving the VM correctly, either there is a failure in a plugin somewhere or due to the VM id isn't getting passed correctly. If this is the first time you encountered the error in the environment it may be better to just do a fresh install.