I'm stumped for now 
The next step would now be to check the vCenter logs from the moment you do that call for one of the datastores that gives the error.
These are the vpxd logs that you find on the vCenter server or via the vSphere Client program.
It could be useful to do the call for a datastore where there is data returned for the QueryMetrics switch and a call for a datastore where the error occurs.
In the vpxd logs you should be able to find the QueryAvailablePerfMetric call for both datastores.
If the method parameters are not listed, change the vCenter logging temporarily to verbose or trivia.
The produced output can be quite lengthy, I'll PM you my mail address to forwrda the logs if you don't want to include or attach them to this thread.