Database with FILESTREAM enabled gives Error 3056 during backup
You have manually create a file in a FS container. Stop doing so.

A FS container is defined here:


  FILESTREAM data must be stored in FILESTREAM filegroups. A FILESTREAM
  filegroup is a special filegroup that contains file system directories
  instead of the files themselves. These file system directories are
  called data containers. Data containers are the interface between
  Database Engine storage and file system storage.


As you see, is nothing but a file system folder. Do not delete or create files in these folder, is simple as that. all interaction must occur via the FILESTREAM API.