diff --git a/api/src/CalDAV/JsBase.php b/api/src/CalDAV/JsBase.php
index d45223103a..9cba134693 100644
--- a/api/src/CalDAV/JsBase.php
+++ b/api/src/CalDAV/JsBase.php
@@ -236,10 +236,10 @@ class JsBase
 				$contact['#'.$name] = null;
 			}
 		}
-		// report not existing cfs to log
+		// error not existing cfs
 		if (($not_existing=array_diff(array_keys($cfs), array_keys($definitions))))
 		{
-			error_log(__METHOD__."() not existing/ignored custom fields: ".implode(', ', $not_existing));
+			throw new JsParseException("Trying to update not existing custom field(s): ".implode(', ', $not_existing));
 		}
 		return $contact;
 	}
diff --git a/api/src/CalDAV/JsCalendar.php b/api/src/CalDAV/JsCalendar.php
index 0125b87bf4..d9353d39d2 100644
--- a/api/src/CalDAV/JsCalendar.php
+++ b/api/src/CalDAV/JsCalendar.php
@@ -110,8 +110,8 @@ class JsCalendar extends JsBase
 				return strpos($key, '/') !== false;
 			}))
 			{
-				// apply patch on JsCard of contact
-				$data = self::patch($data, $old ? self::getJsCalendar($old, false) : [], !$old);
+				// apply patch on JsEvent
+				$data = self::patch($data, $old ? self::getJsCalendar($old, false) : [], !$old || !$strict);
 			}
 
 			if (!isset($data['uid'])) $data['uid'] = null;  // to fail below, if it does not exist
diff --git a/api/src/Contacts/JsContact.php b/api/src/Contacts/JsContact.php
index 4d5fda4c69..6c79233a62 100644
--- a/api/src/Contacts/JsContact.php
+++ b/api/src/Contacts/JsContact.php
@@ -104,7 +104,7 @@ class JsContact extends Api\CalDAV\JsBase
 			}))
 			{
 				// apply patch on JsCard of contact
-				$data = self::patch($data, $old ? self::getJsCard($old, false) : [], !$old);
+				$data = self::patch($data, $old ? self::getJsCard($old, false) : [], !$old || !$strict);
 			}
 
 			if (!isset($data['uid'])) $data['uid'] = null;  // to fail below, if it does not exist
@@ -188,7 +188,7 @@ class JsContact extends Api\CalDAV\JsBase
 						break;
 
 					case 'egroupware.org:customfields':
-						$contact += self::parseCustomfields($value, $strict);
+						$contact += self::parseCustomfields($value);
 						break;
 
 					case 'egroupware.org:assistant':