diff -urN ../z-push-2.6.2+0.orig/src/backend/carddav/carddav.php ./src/backend/carddav/carddav.php
--- ../z-push-2.6.2+0.orig/src/backend/carddav/carddav.php	2021-02-04 03:38:38.000000000 +0200
+++ ./src/backend/carddav/carddav.php	2022-01-29 20:11:11.109686000 +0200
@@ -1323,7 +1323,18 @@
         $raw = $this->server->get(false, false, true);
         if ($raw !== false) {
             $xml = new SimpleXMLElement($raw);
-            foreach ($xml->addressbook_element as $response) {
+            if ($xml->addressbook_element) {
+                $elements = $xml->addressbook_element;
+            } elseif ($xml->element) {
+                $elements = $xml->element;
+            } else {
+                $elements = array();
+            }
+            unset($xml);
+            foreach ($elements as $response) {
+                if (empty($response->url) and !empty($response->id)) {
+                    $response->url = $this->url . $response->id . '/';
+                }
                 if ($this->gal_url !== false) {
                     if (strcmp(urldecode($response->url), $this->gal_url) == 0) {
                         ZLog::Write(LOGLEVEL_DEBUG, sprintf("BackendCardDAV::discoverAddressbooks() Ignoring GAL addressbook '%s'", $this->gal_url));
@@ -1333,7 +1344,7 @@
                 ZLog::Write(LOGLEVEL_DEBUG, sprintf("BackendCardDAV::discoverAddressbooks() Found addressbook '%s'", urldecode($response->url)));
                 $this->addressbooks[] = urldecode($response->url);
             }
-            unset($xml);
+            unset($elements);
         }
     }