diff --git a/src/HOL/Tools/ATP/system_on_tptp.scala b/src/HOL/Tools/ATP/system_on_tptp.scala
--- a/src/HOL/Tools/ATP/system_on_tptp.scala
+++ b/src/HOL/Tools/ATP/system_on_tptp.scala
@@ -1,45 +1,49 @@
 /*  Title:      HOL/Tools/ATP/system_on_tptp.scala
     Author:     Makarius
 
 Support for remote ATPs via SystemOnTPTP.
 */
 
 package isabelle.atp
 
 import isabelle._
 
 import java.net.URL
 
 
 object SystemOnTPTP
 {
   /* requests */
 
   def get_url(options: Options): URL = Url(options.string("SystemOnTPTP"))
 
   def post_request(
     url: URL,
     parameters: List[(String, Any)],
     timeout: Time = HTTP.Client.default_timeout): HTTP.Content =
   {
     val parameters0 =
       List("NoHTML" -> 1, "QuietFlag" -> "-q01")
         .filterNot(p0 => parameters.exists(p => p0._1 == p._1))
-    HTTP.Client.post(url, parameters0 ::: parameters, timeout = timeout, user_agent = "Sledgehammer")
+    try {
+      HTTP.Client.post(url, parameters0 ::: parameters,
+        timeout = timeout, user_agent = "Sledgehammer")
+    }
+    catch { case ERROR(msg) => cat_error("Failed to access SystemOnTPTP server", msg) }
   }
 
 
   /* list systems */
 
   def proper_lines(content: HTTP.Content): List[String] =
     Library.trim_split_lines(content.text).filterNot(_.startsWith("%"))
 
   def list_systems(url: URL): List[String] =
     proper_lines(post_request(url, List("SubmitButton" -> "ListSystems", "ListStatus" -> "READY")))
 
   object List_Systems extends Scala.Fun("SystemOnTPTP.list_systems", thread = true)
   {
     val here = Scala_Project.here
     def apply(url: String): String = cat_lines(list_systems(Url(url)))
   }
 }