src/Pure/Admin/build_status.scala

       val (image_width, image_height) = image_size
       val image_width_stretch = (image_width * data_entry.stretch).toInt
 
       progress.echo("output " + quote(data_name))
 
-      val dir = target_dir + Path.basic(clean_name(data_name))
-      Isabelle_System.make_directory(dir)
+      val dir = Isabelle_System.make_directory(target_dir + Path.basic(clean_name(data_name)))
 
       val data_files =
         (for (session <- data_entry.sessions) yield {
           val csv_file = session.make_csv
           csv_file.write(dir)