Use the view_user_profile reverse view name for the profiles of non-logged in users.
{% extends "base.html" %}
{% load form_helpers %}
{% block content %}
{% if can_claim %}
Propose a claim to work on this task.<br />
{% endif %}
{% if old_claims %}
List of all the claims for the task <a href="{% url view_task task.id %}">{{task.title}}</a><br />
{% for claim in old_claims %}
<hr />
<a href="{% url view_user_profile claim.claimed_by.id %}">{{claim.claimed_by.username}}</a>
on {{claim.claim_datetime|date:"D d M Y"}} at {{claim.claim_datetime|time:"H:i"}} wrote:<br />
{{claim.proposal|linebreaksbr}}<br />
{% endfor %}
{% else %}
{% if task_claimable %}
There are no claims for this task yet.<br />
{% if can_claim %}
Be the first to claim the task.<br />
{% endif %}
{% else %}
The task cannot be claimed at this stage.<br />
{% endif %}
{% endif %}
{% if old_claims and is_creator %}
<hr />
<a href="{% url select_task task.id %}">Select a user to assign the work</a>
{% endif %}
{% if can_claim %}
<hr />
Please note that you can claim only once and so write your
proposal carefully.<br />
{% as_div_form form "Task Claim Form" csrf_token "Request Claim" %}
{% endif %}
{% endblock %}