Don't try to retrieve roles for anonymous posts.

lms/djangoapps/django_comment_client/utils.py

@@ -241,6 +241,8 @@ def permalink(content):
                        args=[content['course_id'], content['commentable_id'], content['thread_id']]) + '#' + content['id']
 
 def extend_content(content):
+    roles = {}
+    if content.get('user_id'):
         user = User.objects.get(pk=content['user_id'])
         roles = dict(('name', role.name.lower()) for role in user.roles.filter(course_id=content['course_id']))
     content_info = {