I’ve just bought a 2TB My Cloud and having this issue on both my iOS devices (iPhone 6 and iPad Air 2).
If I follow the help offered and delete/re-install the App then it’s fixed (device shown is the one I own) until I reboot either device, then it re-appears as a question mark.
Is there any other fixes out there? It’s worrying that my new NAS is doing this… not really something I want to trust with my data if I’m being honest.