OK, the API removeAllSnapshots_Task requires VirtualMachine.State.RemoveSnapshot privilege on vCenter.
If the account you use to login to vRO Java client has an administrator role on vCenter, then it should has this privilege granted by default. I would suggest as a first step to check if this is indeed the case, by going to vSphere Web Client > Administration > Access Control > Roles
The next step would be to check vCenter server logs for additional clues about the error.
Also, if this is a test/dev environment, you can check if updating the vCenter plug-in to a newer version will make any change. Newer builds of the plug-in are available at https://communities.vmware.com/docs/DOC-32872