First thing - when you use {% load comments %} in your template, it uses django.contrib.comments to render the page. When you use {% load threadedcomments_tags %}, it uses django-threadedcomments library.
Which version of Django do you use? It seems like django-threadedcomments is not compatible with Django 1.10. If you want to continue using it, you need to make own adjustments in the django-threadedcomments code, due to change in render_to_string method. Here are some more insights into that: https://github.com/HonzaKral/django-threadedcomments/issues/78
The other solution is to go back to Django 1.9, if you don't mind the version at this point. It works fine.