diff -r fd487d261169 -r 5036edb025b7 src/Pure/Admin/isabelle_cronjob.scala
--- a/src/Pure/Admin/isabelle_cronjob.scala	Sat Apr 04 18:05:37 2020 +0200
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Sat Apr 04 18:13:05 2020 +0200
@@ -514,7 +514,7 @@
         {
           running.partition(_.is_finished) match {
             case (Nil, Nil) =>
-            case (Nil, _ :: _) => Thread.sleep(500); join(running)
+            case (Nil, _ :: _) => Time.seconds(0.5).sleep; join(running)
             case (_ :: _, remaining) => join(remaining)
           }
         }