1) which version is the VC and hosts - 5.0?
2) do you have those datastores under a Folder at the VC?
I think you're hitting a known issue here, where the Datastores have a "parent object", which is a situation that vcops 5.6 cannot handle 
The fix for that will be available with the next maintenance release.
anyway, if your situation is (2), try moving the hosts "out" of the folder(s) - then reports should work.