SQL Server FILESTREAM access denied
The problem is not the user account, as queries are able to run without issue. The problem is firewall-related. Make sure that ports 139 and 445 are open between the application server and the database server, as per http://msdn.microsoft.com/en-us/library/dd283098.aspx:


  To use FILESTREAM in a firewall-protected environment, both the client
  and server must be able to resolve DNS names to the server that
  contains the FILESTREAM files. FILESTREAM requires the Windows
  file-sharing ports 139 and 445 to be open.