I'd like to update the "Admin 404s on users" and dumpdata error:
ContentType matching query does not exist" for people who, like me,
are searching the web for answers.

From the Admin 404s on users discussion (which I cannot seem to reply
to, hence the new thread; eit)
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

"""
from django.contrib.contenttypes.management import create_contenttypes
from django.db.models import get_apps
for app in get_apps():
    create_contenttypes(app, created_models=None, verbosity=2)
"""
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The update is that instead of
    for app in get_apps():
        create_contenttypes(app, created_models=None, verbosity=2)
you can call
    update_all_contenttypes()
or
    for app in get_apps():
        update_contenttypes(app, created_models=None, verbosity=2)

you may find it useful to first import your project settings:
    import sys, os
    from django.core.management import setup_environ
    import settings
    setup_environ(settings)

So what does it mean for permission and content types to be out of
sync? Let's look at the sql:

[1] the two tables we care about are: auth_permission and
django_content_type

mysql> describe auth_permission;
+-----------------+--------------+------+-----+---------
+----------------+
| Field           | Type         | Null | Key | Default |
Extra          |
+-----------------+--------------+------+-----+---------
+----------------+
| id              | int(11)      | NO   | PRI | NULL    |
auto_increment |
| name            | varchar(50)  | NO   |     |
|                |
| content_type_id | int(11)      | NO   | MUL |
|                |
| codename        | varchar(100) | NO   |     |
|                |
+-----------------+--------------+------+-----+---------
+----------------+

mysql> describe django_content_type;
+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| id        | int(11)      | NO   | PRI | NULL    | auto_increment |
| name      | varchar(100) | NO   |     |         |                |
| app_label | varchar(100) | NO   | MUL |         |                |
| model     | varchar(100) | NO   |     |         |                |
+-----------+--------------+------+-----+---------+----------------+

[2] django_content_type.id and auth_permission.content_type_id should
match.

See the pattern in these examples?

mysql> select * from auth_permission;
+----+--------------------------------+-----------------
+---------------------------+
| id | name                           | content_type_id |
codename                  |
+----+--------------------------------+-----------------
+---------------------------+
|  1 | Can add message                |               1 |
add_message               |
|  2 | Can change message             |               1 |
change_message            |
|  3 | Can delete message             |               1 |
delete_message            |
|  4 | Can add group                  |               2 |
add_group                 |
|  5 | Can change group               |               2 |
change_group              |
|  6 | Can delete group               |               2 |
delete_group              |
etc

mysql> select * from django_content_type;
+----+---------------------+--------------+--------------------+
| id | name                | app_label    | model              |
+----+---------------------+--------------+--------------------+
|  1 | message             | auth         | message            |
|  2 | group               | auth         | group              |
|  3 | user                | auth         | user               |
|  4 | permission          | auth         | permission         |
|  5 | content type        | contenttypes | contenttype        |
|  6 | session             | sessions     | session            |
|  7 | site                | sites        | site               |
|  8 | log entry           | admin        | logentry           |
|  9 | room                | bills        | room               |
etc

For me, I had models showing up in auth_permission that didn't exist
anymore. They weren't in django_content_type. Furthermore, I had
duplicate models with wrong content_types, as if a sync had simply
appended auth_permissions instead of updating the models that were
present.

I don't know how it happened. Maybe switching from sqlite3 to mysql?
maybe switching servers. Something let me corrupt or load faulty data
that I couldn't dump!

I deleted and inserted the appropriate rows in auth_permission,
syncdb'ed and reloaded--and voila, it works again.

Probably it's better not to converse with mysql directly. I couldn't
get the python stuff to work. Maybe others will find it useful, or
will update the python code if appropriate. Regardless, it helps to
have a clearer picture of what's going!

ps - The following sql command list is super useful. Props to the
author.
   http://www.pantz.org/software/mysql/mysqlcommands.html

I hope that helps,
diN0bot!