AFAIK there is no RPC backend handling notifications, so disable them…

… completely.

horde/rpc.php

@@ -25,7 +25,7 @@
 // call appInit() until we know which server we are requesting. We  don't
 // initialize the application until after we know the rpc server we want.
 $input = $session_control = $cache_control = null;
-$nocompress = $no_notification = false;
+$nocompress = false;
 $params = array();
 
 /* Look at the Content-type of the request, if it is available, to try
@@ -40,7 +40,6 @@
     $nocompress = true;
     $session_control = 'none';
     $cache_control = 'private';
-    $no_notification = true;
 } elseif (!empty($_SERVER['PATH_INFO']) ||
           in_array($_SERVER['REQUEST_METHOD'], array('DELETE', 'PROPFIND', 'PUT', 'OPTIONS', 'REPORT'))) {
     $serverType = 'Webdav';
@@ -77,7 +76,7 @@
     'nocompress' => $nocompress,
     'session_control' => $session_control,
     'session_cache_limiter' => $cache_control,
-    'nonotificationinit' => $no_notification
+    'nonotificationinit' => true
 ));
 
 $request = $injector->getInstance('Horde_Controller_Request');