I have had no issues with modifying templates within our VMWare environment until our upgrade to ESX 4.1. Since then all our templates errored with:

“Call “PropertyCollector.RetrieveContents” for object “propertyCollector” on vCenter Server “VCentreServer” failed.”

A simple step to correct the issue would be to delete the template from Inventory and re-add it again. This should clear this message and give you back the control of your template.