diff --git a/src/Pure/System/isabelle_process.scala b/src/Pure/System/isabelle_process.scala
--- a/src/Pure/System/isabelle_process.scala
+++ b/src/Pure/System/isabelle_process.scala
@@ -1,62 +1,64 @@
 /*  Title:      Pure/System/isabelle_process.scala
     Author:     Makarius
 
 Isabelle process wrapper.
 */
 
 package isabelle
 
 
 import java.io.{File => JFile}
 
 
 object Isabelle_Process
 {
   def apply(
     session: Session,
     options: Options,
     sessions_structure: Sessions.Structure,
     store: Sessions.Store,
     logic: String = "",
     args: List[String] = Nil,
     modes: List[String] = Nil,
     cwd: JFile = null,
     env: Map[String, String] = Isabelle_System.settings()): Isabelle_Process =
   {
     val channel = System_Channel()
     val process =
       try {
         val channel_options =
           options.string.update("system_channel_address", channel.address).
             string.update("system_channel_password", channel.password)
         ML_Process(channel_options, sessions_structure, store,
           logic = logic, args = args, modes = modes, cwd = cwd, env = env)
       }
       catch { case exn @ ERROR(_) => channel.shutdown(); throw exn }
     process.stdin.close
 
     new Isabelle_Process(session, channel, process)
   }
 }
 
 class Isabelle_Process private(session: Session, channel: System_Channel, process: Bash.Process)
 {
   private val startup_error = Future.promise[String]
 
   session.phase_changed +=
     Session.Consumer(getClass.getName) {
       case Session.Ready =>
         startup_error.fulfill("")
       case Session.Terminated(result) if !result.ok && !startup_error.is_finished =>
-        startup_error.fulfill("Session start failed: return code " + result.rc)
+        val syslog = session.syslog_content()
+        val err = "Session startup failed" + (if (syslog.isEmpty) "" else ":\n" + syslog)
+        startup_error.fulfill(err)
       case _ =>
     }
 
   def startup_join(): Unit =
     startup_error.join match {
       case "" =>
       case msg => session.stop(); error(msg)
     }
 
   session.start(receiver => new Prover(receiver, session.xml_cache, channel, process))
 }
\ No newline at end of file