diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index c916c6f5..429b1218 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -2349,7 +2349,7 @@ $("#rcmfd_new_category").keypress(function(event) {
         }
 
         $attachments = [];
-        $eventid     = 'cal-' . (!empty($event['id']) ? $event['id'] : 'new-event');
+        $eventid     = 'cal-' . (!empty($event['id']) ? $event['id'] : 'new');
 
         if (!empty($_SESSION[self::SESSION_KEY]) && $_SESSION[self::SESSION_KEY]['id'] == $eventid) {
             if (!empty($_SESSION[self::SESSION_KEY]['attachments'])) {
diff --git a/plugins/calendar/calendar_ui.js b/plugins/calendar/calendar_ui.js
index 9cee7279..f1c9d575 100644
--- a/plugins/calendar/calendar_ui.js
+++ b/plugins/calendar/calendar_ui.js
@@ -399,7 +399,7 @@ function rcube_calendar_ui(settings)
         $('#event-alarm').show().find('.event-text').html(Q(event.alarms_text).replace(',', ',<br>'));
       if (calendar.name)
         $('#event-calendar').show().find('.event-text').html(Q(calendar.name)).addClass('cal-'+calendar.id);
-      if (event.categories)
+      if (event.categories && String(event.categories).length)
         $('#event-category').show().find('.event-text').text(event.categories).addClass('cat-'+String(event.categories).toLowerCase().replace(rcmail.identifier_expr, ''));
       if (event.free_busy)
         $('#event-free-busy').show().find('.event-text').text(rcmail.gettext(event.free_busy, 'calendar'));
diff --git a/plugins/libkolab/lib/kolab_attachments_handler.php b/plugins/libkolab/lib/kolab_attachments_handler.php
index bc41e13a..94af9ee0 100644
--- a/plugins/libkolab/lib/kolab_attachments_handler.php
+++ b/plugins/libkolab/lib/kolab_attachments_handler.php
@@ -114,8 +114,9 @@ class kolab_attachments_handler
             $this->rc->upload_progress();
         }
 
-        $recid    = $id_prefix . rcube_utils::get_input_value('_id', rcube_utils::INPUT_GPC);
+        $id       = rcube_utils::get_input_value('_id', rcube_utils::INPUT_GPC);
         $uploadid = rcube_utils::get_input_value('_uploadid', rcube_utils::INPUT_GPC);
+        $recid    = $id_prefix . ($id ?: 'new');
 
         if (empty($_SESSION[$session_key]) || $_SESSION[$session_key]['id'] != $recid) {
             $_SESSION[$session_key] = array();