src/Pure/System/options.scala

--- a/src/Pure/System/options.scala	Sat Oct 03 23:01:40 2020 +0100
+++ b/src/Pure/System/options.scala	Mon Oct 05 21:15:58 2020 +0200
@@ -403,7 +403,7 @@
       changed.sortBy(_._1)
         .map({ case (x, y, z) => x + " = " + Outer_Syntax.quote_string(y) + z + "\n" }).mkString
 
-    Isabelle_System.mkdirs(file.dir)
+    Isabelle_System.make_directory(file.dir)
     File.write_backup(file, "(* generated by Isabelle " + Date.now() + " *)\n\n" + prefs)
   }
 }