[Fixed]-Django admin returns 404 on POST, 200 on GET

Have you activated middleware (Internationalization: in URL patterns)?

# settings.py
MIDDLEWARE = [
...
'django.middleware.locale.LocaleMiddleware'
...
]

I never got to the bottom of this, but I have developed a workaround that allows the forms to function.

1. Edit _get_response(self, request) in django.core.handlers.base. Change resolver_match = resolver.resolve(request.path_info) to resolver_match = resolver.resolve(request.path).

2. Add this middleware (adjust to your exact needs):

    class ImageField404Middleware:
        def __init__(self, get_response):
            self.get_response = get_response
   
        def __call__(self, request):
            response = self.get_response(request)
   
            if (request.method == 'POST' and request.user.is_superuser and response.status_code == 302
                    and request.get_full_path().startswith('/pathtoadmin/')):
                post_messages = get_messages(request)
                for message in post_messages:
                    if ('was added successfully' in message.message or 'was changed successfully' in message.message
                            and message.level == message_levels.SUCCESS):
                        messages.success(request, message.message)
                        redirect_url = request.get_full_path()
                        if '_addanother' in request.POST:
                            redirect_url = re.sub(r'[^/]*/[^/]*/$', 'add/', redirect_url)
                        elif '_save' in request.POST:
                            redirect_url = re.sub(r'[^/]*/[^/]*/(\?.*)?$', '', redirect_url) 
                            if '_changelist_filters' in request.GET:
                                preserved_filters = parse.parse_qsl(request.GET['_changelist_filters'])
                                redirect_url += '?' + parse.urlencode(preserved_filters)         
                        elif '_continue' in request.POST:
                            redirect_url_search = re.search(r'((?<=href=)[^>]*)', message.message)    
                            if redirect_url_search:                                                  
                                redirect_url = redirect_url_search.group(0)                                                  
                                redirect_url = re.sub(r'[\\"]*', '', redirect_url).replace('/pathtoadmin/pathtoadmin/', '/pathtoadmin/')
                        return HttpResponseRedirect(redirect_url)
   
            return response
   

Not ideal by any means, but works for me at the moment.

I believe these problems may, in part at least, be caused by ModSecurity Apache. Some problems I had went away after the hosting provider later reconfigured this.

You can read more details here: https://medium.com/@mnydigital/how-to-resolve-django-admin-404-post-error-966ce0dcd39d