diff --git a/plugins/kolab_delegation/kolab_delegation_engine.php b/plugins/kolab_delegation/kolab_delegation_engine.php
index a286e4b2..48c934b2 100644
--- a/plugins/kolab_delegation/kolab_delegation_engine.php
+++ b/plugins/kolab_delegation/kolab_delegation_engine.php
@@ -109,7 +109,7 @@ class kolab_delegation_engine
         $folders     = $update ? $this->list_folders($uid) : array();
 
         foreach ($acl as $folder_name => $rights) {
-            $r = $right_types[$rights];
+            $r = $right_types[$rights] ?? null;
             if ($r) {
                 $storage->set_acl($folder_name, $uid, $r);
             }
@@ -123,7 +123,7 @@ class kolab_delegation_engine
         }
 
         foreach ($folders as $folder_name => $folder) {
-            if ($folder['rights']) {
+            if (!empty($folder['rights'])) {
                 $storage->delete_acl($folder_name, $uid);
             }
         }
diff --git a/plugins/kolab_folders/kolab_folders.php b/plugins/kolab_folders/kolab_folders.php
index a6f7fb0f..340901fe 100644
--- a/plugins/kolab_folders/kolab_folders.php
+++ b/plugins/kolab_folders/kolab_folders.php
@@ -234,7 +234,7 @@ class kolab_folders extends rcube_plugin
         $subtype = '';
         if (strlen($mbox)) {
             list($ctype, $subtype) = $this->get_folder_type($mbox);
-            if (strlen($args['parent_name']) && $subtype == 'default') {
+            if (isset($args['parent_name']) && strlen($args['parent_name']) && $subtype == 'default') {
                 $subtype = ''; // there can be only one
             }
         }
diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index 893af60b..9f4791e5 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -545,7 +545,7 @@ class kolab_storage
             }
         }
 
-        if (!empty($options) && ($options['protected'] || $options['norename'])) {
+        if (!empty($options) && (!empty($options['protected']) || !empty($options['norename']))) {
             $folder = $oldfolder;
         }
         else if (strlen($parent)) {
@@ -1101,13 +1101,13 @@ class kolab_storage
                 if (!empty($parents)) {
                     $parents = array_reverse($parents);
                     foreach ($parents as $parent) {
-                        $parent_node = $refs[$parent->parent] ?: $tree;
+                        $parent_node = !empty($refs[$parent->parent]) ? $refs[$parent->parent] : $tree;
                         $parent_node->children[] = $parent;
                         $_folders[] = $parent;
                     }
                 }
 
-                $parent_node = $refs[$folder->parent] ?: $tree;
+                $parent_node = !empty($refs[$folder->parent]) ? $refs[$folder->parent] : $tree;
                 $parent_node->children[] = $folder;
             }
 
diff --git a/plugins/libkolab/lib/kolab_utils.php b/plugins/libkolab/lib/kolab_utils.php
index 571bf286..d80d0140 100644
--- a/plugins/libkolab/lib/kolab_utils.php
+++ b/plugins/libkolab/lib/kolab_utils.php
@@ -48,7 +48,7 @@ class kolab_utils
 
         // create form output
         foreach ($form as $tab) {
-            if (is_array($tab['fields']) && empty($tab['content'])) {
+            if (isset($tab['fields']) && is_array($tab['fields']) && empty($tab['content'])) {
                 $table = new html_table(array('cols' => 2, 'class' => 'propform'));
                 foreach ($tab['fields'] as $col => $colprop) {
                     $label = !empty($colprop['label']) ? $colprop['label'] : $rcmail->gettext("$domain.$col");
@@ -88,6 +88,10 @@ class kolab_utils
                 'name'    => $folder
         ));
 
-        return $acl['form']['sharing']['content'] ?: html::div('hint', $rcmail->gettext('libkolab.aclnorights'));
+        if (!empty($acl['form']['sharing']['content'])) {
+            return $acl['form']['sharing']['content'];
+        }
+        
+        return html::div('hint', $rcmail->gettext('libkolab.aclnorights'));
     }
 }