As I can see in above error json.dump() fails to create JSON from object passed in /edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/xblock/mixins.py
Mr. @omar even highlighted something similar here : Translate the default XBlock data - #3 by omar
Will it be right to use following code inside xblock/mixins.py
from rest_framework.utils.encoders import JSONEncoder
value = json.dumps(..., cls=JSONEncoder)
For file “/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/xblock/mixins.py”, line 74, in wrapper
return Response(json.dumps(response), content_type=‘application/json’, charset=‘utf8’)