django cms - invalid block tag endblock
I figured it out. The problem was in the template. I was unaware that sekizai tags couldn't work inside block tags. From the sekizai documentation:
The tag must be in the base template. It cannot be used in an included template. The tag must not be placed within a block tag (a template tag with an end tag, for example {% block name %}...{% endblock %}).
For future reference to anyone who may encounter this. This was the offending code:
{% block extra_style %}{% render_block "css" %}{% endblock %}