I have a django app which was running on 1.4.2 version and working completely fine, but recently i updated it to django 1.6.5 and facing some wierd
After analyzing the traceback, it seems that the JSONEncoder can not serialize the instance of your Client model. Generally, you got such error if you try to serialize a model related to other models (Many2ManyField, etc.) using json or simplejson libraries.
See this https://docs.djangoproject.com/en/dev/topics/serialization/, You can also use some 3rd-party packages such as DjangoRestFramework depending on your needs.