I have a similar issue with my Active Email count in Interaction Statistics in the Business Manager. I have a switchover pair in my Test environment and my counts go NA when I'm on the other server. I have over a thousand emails in the test server.
Business Manager log: Uri=inin.system.interaction:ActiveEmails at 10/5/2017 1:46:00 PM - value is Int: Id=ININ.System.Interaction:ActiveEmails,
Val=0, IsNull=True, IsError=False. Adding to 1 minute and 5 minute queues.
Thanks
Guy
2017R2_P6 in Test.