src/Pure/System/classpath.scala

   val class_loader: ClassLoader =
   {
     val this_class_loader = this.getClass.getClassLoader
     if (dynamic_jars.isEmpty) this_class_loader
     else {
-      new URLClassLoader(dynamic_jars.map(File.url).toArray, this_class_loader) {
+      val dynamic_jars_url = dynamic_jars.map(file => File.url(file).java_url)
+      new URLClassLoader(dynamic_jars_url.toArray, this_class_loader) {
         override def finalize(): Unit = {
           for (jar <- dynamic_jars) {
             try { jar.delete() }
             catch { case _: Throwable => }
           }