diff --git a/api/js/jsapi/egw_json.js b/api/js/jsapi/egw_json.js
index b79517f718..64f2247b59 100644
--- a/api/js/jsapi/egw_json.js
+++ b/api/js/jsapi/egw_json.js
@@ -149,9 +149,19 @@ egw.extend('json', egw.MODULE_WND_LOCAL, function(_app, _wnd)
 			this.egw.message.call(this.egw, this.egw.lang('A request to the EGroupware server returned with an error')+': '+_xmlhttp.statusText+' ('+_xmlhttp.status+
 				")\n\n"+this.egw.lang('Please reload the EGroupware desktop (F5 / Cmd+r).')+"\n"+
 				this.egw.lang('If the error persists, contact your administrator for help and ask to check the error-log of the webserver.')+
-				"\n\nURL: "+this.url+"\n"+(_xmlhttp.getAllResponseHeaders() ? _xmlhttp.getAllResponseHeaders().match(/^Date:.*$/m)[0]:''));
+				"\n\nURL: "+this.url+"\n"+(_xmlhttp.getAllResponseHeaders() ? _xmlhttp.getAllResponseHeaders().match(/^Date:.*$/mi)[0]:'')+
+				// if EGroupware send JSON payload with error, errno show it here too
+				(_err === 'error' && _xmlhttp.status === 400 && typeof _xmlhttp.responseJSON === 'object' && _xmlhttp.responseJSON.error ?
+				"\nError: "+_xmlhttp.responseJSON.error+' ('+_xmlhttp.responseJSON.errno+')' : ''));
 
-			this.egw.debug('error', 'Ajax request to', this.url, ' failed: ', _err, _xmlhttp.status, _xmlhttp.statusText);
+			this.egw.debug('error', 'Ajax request to', this.url, ' failed: ', _err, _xmlhttp.status, _xmlhttp.statusText, _xmlhttp.responseJSON);
+
+			// check of unparsable JSON on server-side, which might be caused by some network problem --> resend max. twice
+			if (_err === 'error' && _xmlhttp.status === 400 && typeof _xmlhttp.responseJSON === 'object' &&
+				_xmlhttp.responseJSON.errno && _xmlhttp.responseJSON.error.substr(0, 5) === 'JSON ')
+			{
+				// ToDo: resend request max. twice
+			}
 		}
 	};
 
diff --git a/api/src/Json/Request.php b/api/src/Json/Request.php
index f913ee076b..3a2ace84f9 100644
--- a/api/src/Json/Request.php
+++ b/api/src/Json/Request.php
@@ -54,23 +54,36 @@ class Request
 	 *
 	 * @param string menuaction to call
 	 * @param string $input_data is the RAW input data as it was received from the client
+	 * @throws \InvalidArgumentException if JSON can not be parsed (json_last_error())
+	 *  or did not contain request[parameters] array (999)
 	 */
 	public function parseRequest($menuaction, $input_data)
 	{
 		// Remember that we currently are in a JSON request - e.g. used in the redirect code
 		self::$_hadJSONRequest = true;
 
-		if (get_magic_quotes_gpc()) $input_data = stripslashes($input_data);
-
-		$json_data = json_decode($input_data,true);
-		if (is_array($json_data) && isset($json_data['request']) && isset($json_data['request']['parameters']) && is_array($json_data['request']['parameters']))
+		// no or empty payload is eg. used by dynamicly loading tree nodes (uses just GET parameters)
+		if (!isset($input_data) || $input_data === '')
 		{
-			//error_log(__METHOD__.__LINE__.array2string($json_data['request']).function_backtrace());
-			$parameters =& $json_data['request']['parameters'];
+			$parameters = array();
 		}
 		else
 		{
-			$parameters = array();
+			if (get_magic_quotes_gpc()) $input_data = stripslashes($input_data);
+
+			if (($json_data = json_decode($input_data,true)) === null && json_last_error() !== JSON_ERROR_NONE)
+			{
+				throw new \InvalidArgumentException('JSON '.json_last_error_msg(), json_last_error());
+			}
+			elseif (is_array($json_data) && isset($json_data['request']) && isset($json_data['request']['parameters']) && is_array($json_data['request']['parameters']))
+			{
+				//error_log(__METHOD__.__LINE__.array2string($json_data['request']).function_backtrace());
+				$parameters =& $json_data['request']['parameters'];
+			}
+			else
+			{
+				throw new \InvalidArgumentException('Missing request:parameters object', 999);
+			}
 		}
 		// do we have a single request or an array of queued requests
 		if ($menuaction == 'api.queue')
diff --git a/json.php b/json.php
index 4410cf702f..859572a2a1 100644
--- a/json.php
+++ b/json.php
@@ -70,8 +70,11 @@ function ajax_exception_handler($e)
 // set our own exception handler, to not get the html from eGW's default one
 set_exception_handler('ajax_exception_handler');
 
-if (isset($_GET['menuaction']))
-{
+try {
+	if (!isset($_GET['menuaction']))
+	{
+		throw new InvalidArgumentException('Missing menuaction GET parameter', 998);
+	}
 	if (strpos($_GET['menuaction'],'::') !== false && strpos($_GET['menuaction'],'.') === false)	// static method name app_something::method
 	{
 		@list($className,$functionName,$handler) = explode('::',$_GET['menuaction']);
@@ -127,5 +130,13 @@ if (isset($_GET['menuaction']))
 	Json\Response::get();
 	exit();
 }
+// missing menuaction GET parameter or request:parameters object or unparsable JSON
+catch (\InvalidArgumentException $e) {
+	if (isset($json)) $json->isJSONRequest(false);	// no regular json request processing
 
-throw new Json\Exception($_SERVER['PHP_SELF'] . ' Invalid AJAX JSON Request');
+	// give a proper HTTP status 400 Bad Request with some JSON payload explaining the problem
+	http_response_code(400);
+	header('Content-Type: application/json');
+	echo json_encode(array('error' => $e->getMessage(), 'errno' => $e->getCode()));
+}
+// other exceptions are handled by our ajax_exception_handler sending them back as alerts to client-side
\ No newline at end of file