diff --git a/src/java/davmail/caldav/CaldavConnection.java b/src/java/davmail/caldav/CaldavConnection.java
index 93c3765b..96d45785 100644
--- a/src/java/davmail/caldav/CaldavConnection.java
+++ b/src/java/davmail/caldav/CaldavConnection.java
@@ -1118,6 +1118,12 @@ public class CaldavConnection extends AbstractConnection {
         } else if (e instanceof HttpPreconditionFailedException) {
             sendErr(HttpStatus.SC_PRECONDITION_FAILED, message);
         } else {
+            // workaround for Lightning bug: sleep for 1 second
+            try {
+                Thread.sleep(1000);
+            } catch (InterruptedException ie) {
+                // ignore
+            }
             sendErr(HttpStatus.SC_SERVICE_UNAVAILABLE, message);
         }
     }
diff --git a/src/site/resources/images/osxgatekeeper.png b/src/site/resources/images/osxgatekeeper.png
new file mode 100644
index 00000000..e51f0966
Binary files /dev/null and b/src/site/resources/images/osxgatekeeper.png differ