Got it to happen inside the admin interface. It looks like, at least in this case, it's some kind of cache. I don't see the cache stuff on the user's listing page. I'll see if I can put a condensed version of the error here:
Error during template rendering
In template.../python3.6/site-packages/django/contrib/admin/templates/admin/base.html, error at line 0
'%(value)s' is not a valid UUID.
1 {% load i18n static %}<!DOCTYPE html>
2 {% get_current_language as LANGUAGE_CODE %}{% get_current_language_bidi as LANGUAGE_BIDI %}
3 <html lang="{{ LANGUAGE_CODE|default:"en-us" }}" {% if LANGUAGE_BIDI %}dir="rtl"{% endif %}>
4 <head>
5 <title>{% block title %}{% endblock %}</title>
6 <link rel="stylesheet" type="text/css" href="{% block stylesheet %}{% static "admin/css/base.css" %}{% endblock %}" />
7 {% block extrastyle %}{% endblock %}
8 {% if LANGUAGE_BIDI %}<link rel="stylesheet" type="text/css" href="{% block stylesheet_rtl %}{% static "admin/css/rtl.css" %}{% endblock %}" />{% endif %}
9 {% block extrahead %}{% endblock %}
10 {% block responsive %}
Traceback
.../django/db/models/fields/related_descriptors.py in __get__
rel_obj = self.field.get_cached_value(instance) ...
.../django/db/models/fields/mixins.py in get_cached_value
return instance._state.fields_cache[cache_name] ...
During handling of the above exception ('package'), another exception occurred:
.../django/db/models/fields/__init__.py in to_python
return uuid.UUID(value) ...
/usr/local/lib/python3.6/uuid.py in __init__
hex = hex.replace('urn:', '').replace('uuid:', '') ...
During handling of the above exception ('UUID' object has no attribute 'replace'), another exception occurred:
.../django/core/handlers/exception.py in inner....