x-file-dialog uses GetOpenFileName to get a file name.
If you have a buffer that is not visiting any file, when you call save-buffer through a tool-bar button, x-file-dialog gets called.
However, with GetOpenFileName, you must select an existing file.

We can fix this bug by minimum change.

in x-file-dialog (defined in win32fns.c)

change the line:

    file_opened = GetOpenFileName (file_details);

to

    if (!NILP (mustmatch))
      {
        file_opened = GetOpenFileName (file_details);
      }
    else
      {
        file_opened = GetSaveFileName (file_details);
      }