User Configuration ->Policies -> Administrative Templates -> Start Menu and Taskbar-> Remove Run menu from Start Menu
I had this policy applied to a Remote Desktop Session Host through Loopback in a user session lockdown policy.
When this policy was applied, I could not access any network share on my 2012 R2 file servers and received the following error message:
Accessing the resource \\fileserver\share has been disallowed.
I could received the same error when attempting to access the file server shares through a DFS namespace.
After removing this policy setting from my RD Session Hosts, the issue was resolved.