diff --git a/lib/Kolab/CalDAV/CalendarBackend.php b/lib/Kolab/CalDAV/CalendarBackend.php
--- a/lib/Kolab/CalDAV/CalendarBackend.php
+++ b/lib/Kolab/CalDAV/CalendarBackend.php
@@ -112,13 +112,13 @@
     public function get_storage_folder($id)
     {
         // resolve alias name
-        if ($this->aliases[$id]) {
+        if ($this->aliases[$id] ?? false) {
             $id = $this->aliases[$id];
         }
 
         if ($this->folders[$id]) {
             DAVBackend::check_storage_folder($this->folders[$id]);
-            return $this->folders[$id];
+            return $this->folders[$id] ?? null;
         }
         else {
             return DAVBackend::get_storage_folder($id, '');
@@ -180,11 +180,11 @@
         $id = $calendarUri;
 
         // resolve aliases (calendar by folder name)
-        if ($this->aliases[$calendarUri]) {
+        if ($this->aliases[$calendarUri] ?? false) {
             $id = $this->aliases[$calendarUri];
         }
 
-        if ($this->calendars[$id] && empty($this->calendars[$id]['principaluri'])) {
+        if (($this->calendars[$id] ?? false) && empty($this->calendars[$id]['principaluri'])) {
             $this->calendars[$id]['principaluri'] = 'principals/' . HTTPBasic::$current_user;
         }
 
diff --git a/lib/Kolab/DAV/Auth/HTTPBasic.php b/lib/Kolab/DAV/Auth/HTTPBasic.php
--- a/lib/Kolab/DAV/Auth/HTTPBasic.php
+++ b/lib/Kolab/DAV/Auth/HTTPBasic.php
@@ -74,7 +74,7 @@
             }
 
             // LDAP server failure... send 503 error
-            if ($auth['kolab_ldap_error']) {
+            if ($auth['kolab_ldap_error'] ?? false) {
                 throw new ServiceUnavailable('The service is temporarily unavailable (LDAP failure)');
             }
         }
@@ -85,8 +85,10 @@
             $_SESSION['kolab_auth_vars'] = $auth['vars'];
         }
 
+        $error = null;
+        $error_str = null;
         // authenticate user against the IMAP server
-        $user_id = $auth['abort'] ? 0 : $this->_login($auth['user'], $auth['pass'], $auth['host'], $error);
+        $user_id = ($auth['abort'] ?? false) ? 0 : $this->_login($auth['user'], $auth['pass'], $auth['host'], $error);
 
         if ($user_id) {
             self::$current_user = $auth['user'];
@@ -181,6 +183,7 @@
 
         // parse $host
         $a_host = parse_url($host);
+        $port = null;
         if ($a_host['host']) {
             $host = $a_host['host'];
             $ssl = (isset($a_host['scheme']) && in_array($a_host['scheme'], array('ssl','imaps','tls'))) ? $a_host['scheme'] : null;