diff -r aa4987fede52 -r e00a24b711a0 server/python/django2/renkanmanager/serializers.py
--- a/server/python/django2/renkanmanager/serializers.py	Tue Jun 07 10:46:20 2016 +0200
+++ b/server/python/django2/renkanmanager/serializers.py	Mon Jun 13 14:23:58 2016 +0200
@@ -25,15 +25,19 @@
     modification_date = serializers.SerializerMethodField("get_current_revision_modification_date")
     create_new_revision = serializers.BooleanField(write_only=True, required=False) # only used for updating
     validation_timestamp = serializers.CharField(write_only=True, required=False) # only used for updating is a content json is not provided
-    
+
     # ADD ERROR HANDLING
     def get_current_revision_last_updator(self, renkan):
-        return getattr(renkan.current_revision.last_updated_by, RENKAN_USER_DISPLAY_FIELD)
-    
-    
+        last_updator = renkan.current_revision.last_updated_by
+        if last_updator:
+            return getattr(last_updator, RENKAN_USER_DISPLAY_FIELD)
+        else:
+            return ''
+
+
     def get_current_revision_modification_date(self, renkan):
         return renkan.current_revision.modification_date
-    
+
     @transaction.atomic
     def create(self, validated_data):
         """
@@ -52,7 +56,7 @@
             )
         except ValidationError as ve:
             raise serializers.ValidationError(ve.args[0])
-    
+
     @transaction.atomic
     def update(self, renkan, validated_data):
         """
@@ -80,18 +84,18 @@
         except ValidationError as ve:
             raise serializers.ValidationError(str(ve.args[0]))
         return renkan
-    
+
     def validate_workspace_id(self, value):
         if self.instance is not None:
             raise serializers.ValidationError("You cannot update workspace_guid")
         return value
-    
+
     def validate_source_revision_id(self, value):
         if self.instance is not None:
             raise serializers.ValidationError("You cannot update source_revision_guid")
         return value
-    
-    def validate_content(self, value):  
+
+    def validate_content(self, value):
         try:
             loaded_json = json.loads(value)
         except ValueError:
@@ -103,9 +107,10 @@
         if (not "views" in loaded_json):
             raise serializers.ValidationError("Content requires a 'views' entry")
         return value
-    
+
     def validate_validation_timestamp(self, value):
-        logger.debug("%r", value)
+        logger.debug("%r < %r", dateparse.parse_datetime(value), self.get_current_revision_modification_date(self.instance))
+
         if self.instance and dateparse.parse_datetime(value) < self.get_current_revision_modification_date(self.instance):
             raise serializers.ValidationError("Invalid timestamp was provided")
         return value
@@ -121,19 +126,19 @@
     revision_created_by = serializers.SlugRelatedField(source="creator", slug_field=RENKAN_USER_DISPLAY_FIELD, read_only=True)
     revision_last_updated_by = serializers.SlugRelatedField(source="last_updated_by", slug_field=RENKAN_USER_DISPLAY_FIELD, read_only=True)
     revision_modification_date = serializers.ReadOnlyField(source="modification_date")
-    
+
     def get_related_workspace_guid(self, revision):
         if revision.parent_renkan.workspace:
             return revision.parent_renkan.workspace.workspace_guid
         else:
             return ''
-    
+
     def get_related_renkan_creator(self, revision):
         return getattr(revision.parent_renkan.creator, RENKAN_USER_DISPLAY_FIELD)
-    
+
     def get_related_renkan_creation_date(self, revision):
         return revision.parent_renkan.creation_date
-    
+
 
 class WorkspaceSerializer(serializers.Serializer):
     id = serializers.ReadOnlyField(source="workspace_guid")
@@ -141,15 +146,15 @@
     creation_date = serializers.ReadOnlyField()
     renkan_count = serializers.ReadOnlyField()
     title = serializers.CharField()
-    
+
     def create(self, validated_data):
         creator = validated_data.get('creator')
         title = validated_data.get('title', '')
         workspace = Workspace.objects.create(creator=creator, title=title)
         workspace.save()
         return workspace
-    
+
     def update(self, workspace, validated_data):
         workspace.title = validated_data.get('title', '')
         workspace.save()
-        return workspace
\ No newline at end of file
+        return workspace