I had this happen last night with a movie I rented via Google Play Movies on my Shield (2017). I closed the app completely and went back in and it worked fine.
Go into the Account area on the device and make sure the google account logged into the device has GPM Sync turned on. There are sync sliders for all google services and GPM was the only one NOT turned on. I had this same issue on my Shield.
If you've done a hard reset and it's still doing this, you might have to factory reset the device.
I don't have experience with that box exactly, but if you contact Google support I can almost guarantee they'll have you do this.