Unbreak the API in Magento 1.9.3. Not convinced this is correct, as nested properties in objects and arrays do not get the XML escaping magic applied to them. At least this is no less broken than it was before 1.9.3!

unbreak-api.diff

diff --git a/app/code/core/Mage/Api/Model/Server/Handler/Abstract.php b/app/code/core/Mage/Api/Model/Server/Handler/Abstract.php
index bfc6763..2566340 100644
--- a/app/code/core/Mage/Api/Model/Server/Handler/Abstract.php
+++ b/app/code/core/Mage/Api/Model/Server/Handler/Abstract.php
@@ -556,12 +556,16 @@ abstract class Mage_Api_Model_Server_Handler_Abstract
      * @param array $result
      * @return mixed
      */
-    public function processingMethodResult(array $result)
+    public function processingMethodResult($result)
     {
-        foreach ($result as &$row) {
-            if (!is_null($row) && !is_bool($row) && !is_numeric($row)) {
-                $row = $this->processingRow($row);
+        if (is_array($result)) {
+            foreach ($result as &$row) {
+                if (!is_null($row) && !is_bool($row) && !is_numeric($row)&& !is_object($row) && !is_array($row)) {
+                    $row = $this->processingRow($row);
+                }
             }
+        } elseif (!is_null($result) && !is_bool($result) && !is_numeric($result) && !is_object($row) && !is_array($row)) {
+            $result = $this->processingRow($result);
         }
         return $result;
     }