diff --git a/calendar/inc/class.calendar_groupdav.inc.php b/calendar/inc/class.calendar_groupdav.inc.php
index 8106c36e04..1cd8849707 100644
--- a/calendar/inc/class.calendar_groupdav.inc.php
+++ b/calendar/inc/class.calendar_groupdav.inc.php
@@ -774,6 +774,8 @@ class calendar_groupdav extends groupdav_handler
 				if ($schedule_tag_match !== $schedule_tag)
 				{
 					if ($this->debug) error_log(__METHOD__."(,,$user) schedule_tag missmatch: given '$schedule_tag_match' != '$schedule_tag'");
+					// honor Prefer: return=representation for 412 too (no need for client to explicitly reload)
+					$this->check_return_representation($options, $id, $user);
 					return '412 Precondition Failed';
 				}
 			}
@@ -891,6 +893,8 @@ class calendar_groupdav extends groupdav_handler
 			}
 			elseif ($cal_id === 0)	// etag failure
 			{
+				// honor Prefer: return=representation for 412 too (no need for client to explicitly reload)
+				$this->check_return_representation($options, $id, $user);
 				return '412 Precondition Failed';
 			}
 			else
diff --git a/phpgwapi/inc/class.groupdav_handler.inc.php b/phpgwapi/inc/class.groupdav_handler.inc.php
index 1688453c67..d3f15cdea8 100644
--- a/phpgwapi/inc/class.groupdav_handler.inc.php
+++ b/phpgwapi/inc/class.groupdav_handler.inc.php
@@ -309,6 +309,8 @@ abstract class groupdav_handler
 				if ($this->http_if_match !== $etag)
 				{
 					if ($this->debug) error_log(__METHOD__."($method,path=$options[path],$id) HTTP_IF_MATCH='$_SERVER[HTTP_IF_MATCH]', etag='$etag': 412 Precondition failed".array2string($entry));
+					// honor Prefer: return=representation for 412 too (no need for client to explicitly reload)
+					$this->check_return_representation($options, $id);
 					return '412 Precondition Failed';
 				}
 			}
@@ -327,6 +329,8 @@ abstract class groupdav_handler
 				if ($method == 'PUT' && ($if_none_match == '*' || $if_none_match == $etag))
 				{
 					if ($this->debug) error_log(__METHOD__."($method,,$id) HTTP_IF_NONE_MATCH='$_SERVER[HTTP_IF_NONE_MATCH]', etag='$etag': 412 Precondition failed");
+					// honor Prefer: return=representation for 412 too (no need for client to explicitly reload)
+					$this->check_return_representation($options, $id);
 					return '412 Precondition Failed';
 				}
 			}
@@ -344,7 +348,8 @@ abstract class groupdav_handler
 	 */
 	public function check_return_representation($options, $id, $user=null)
 	{
-		if (isset($_SERVER['HTTP_PREFER']) && in_array('return=representation', explode(',', $_SERVER['HTTP_PREFER'])))
+		//error_log(__METHOD__."(, $id, $user) start ".function_backtrace());
+		if (isset($_SERVER['HTTP_PREFER']) && in_array('return=representation', preg_split('/, ?/', $_SERVER['HTTP_PREFER'])))
 		{
 			if ($_SERVER['REQUEST_METHOD'] == 'POST')
 			{
@@ -355,6 +360,11 @@ abstract class groupdav_handler
 				}
 				header('Content-Location: '.$location);
 			}
+
+			// remove If-Match or If-None-Match headers, otherwise HTTP status 412 goes into endless loop!
+			unset($_SERVER['HTTP_IF_MATCH']);
+			unset($_SERVER['HTTP_IF_NONE_MATCH']);
+
 			if (($ret = $this->get($options, $id ? $id : $this->new_id, $user)) && !empty($options['data']))
 			{
 				header('Content-Length: '.$this->groupdav->bytes($options['data']));
@@ -362,6 +372,7 @@ abstract class groupdav_handler
 				echo $options['data'];
 			}
 		}
+		//error_log(__METHOD__."(, $id, $user) returning ".array2string($ret));
 		return $ret;
 	}