Hi Richard,
Which version of the PI SDK is installed on the PI Server where PCo is installed? The PI SDK is a required component of the PCo OSISoft PI Source Agent.
Please set the Logging level of the PCo Agent Instance to Verbose and then re-execute the MII TagQuery by opening the TagQuery Details page. PCo will log the incoming query from MII so that we can see what is being sent from MII, and log the details of the responses from PI.
Regards, Steve