Not an answer just an update:
{% comment %}
<p><a href="{% url img_list_images %}"> Imagelist</a></p>
{% uncomment %}
is not correct.
Correctly commented out the lines look like
{% comment %}
<p><a href="{% url img_list_images %}"> Imagelist</a></p>
{% endcomment %}
The wrong comment out brace caused the inlude file to be ignored at all, so no error was generated. If the comment brace is set correctly the line
<p><a href="{% url img_list_images %}"> Imagelist</a></p>
will always cause an error, even if I change names.