Index: univention-directory-manager-modules/modules/univention/admin/handlers/__init__.py
===================================================================
--- univention-directory-manager-modules/modules/univention/admin/handlers/__init__.py	(revision 80962)
+++ univention-directory-manager-modules/modules/univention/admin/handlers/__init__.py	(working copy)
@@ -837,7 +837,10 @@
 		# Make sure all default values are set...
 		for name, p in self.descriptions.items():
 			# ... if property has no option or any required option is currently enabled
-			if self.has_key(name) and p.default(self):  # noqa: W601
+			# Unset extending attributes with datatype boolean are interpreted as false
+			# and should not be set to their default value during object modification.
+			existing_boolean = self.exists() and p.syntax.type == 'boolean'
+			if self.has_key(name) and p.default(self) and not existing_boolean:  # noqa: W601
 				self[name]  # __getitem__ sets default value
 
 	def _ldap_object_classes(self, ml):