We discovered the cause of the error.
We were configuring a new datacenter within the vCenter server - total of two datacenters. The second datacenter had been created, but no ESX hosts or clusters were created within the datacenter.
The WFA (2.1) datasource for vCenter returned the error above because there were no items within the datacenter. Removing the datacenter cleared the problem.
We have reported this as a bug to be fixed in the vCenter datasource code.
Phil