After some more digging into this I found a way to reproduce it using a much simpler URL, like the following (but with the real name instead of example.org):
https://example.org/mailman3/postorius/lists/?items=x
The error comes when Django is trying to process the template django-mailman3/django_mailman3/templates/django_mailman3/paginator/pagination.html which contains two for-loops of the following kind:
{% for key, value in request.GET.items %} [... doing something with key and value here ...] {% endfor %}
It seems like Django then gets confused by the different "items" variables involved, I think the value from the "?items=x" part of the request URL gets used instead of the "request.GET.items" (both are called "items" but they contain different things.
I don't know if the for-loops in that template could be done differently to avoid this.
There is something similar at https://github.com/django/django/blob/main/django/views/templates/technical_... where the corresponding for-loop looks like this instead:
{% for k, v in request_GET_items %} [...] {% endfor %}
So there they write "request_GET_items" instead of "request.GET.items" for some reason. I don't know if doing it like that is possible in case of the mailman template, just mentioning it because it looks like it could be another way to achieve the same thing.
I wrote a question about this in the Django forum also, here: https://forum.djangoproject.com/t/getting-error-need-2-values-to-unpack-in-f...
Hoping for an answer there to help figure out if this should be seen as a bug in Django or not.
/ Elias