Question: I am using MongoDB(Mongo Atlas) in my Django app. All was working fine till yesterday. But today, when I ran the server, it is showing me the following error on console Exception in thread django-main-thread: Traceback (most recent call last): File "c:\users\admin\appdata\local\programs\python\python39\lib\threading.py", line 973, in _bootstrap_inner self.run() File "c:\users\admin\appdata\local\programs\python\python39\lib\threading.py", line 910, in run self._target(*self._args, **self._kwargs) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\utils\autoreload.py", line 64, in wrapper fn(*args, **kwargs) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\core\management\commands\runserver.py", line 121, in inner_run self.check_migrations() File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\core\management\base.py", line 486, in check_migrations executor = MigrationExecutor(connections[DEFAULT_DB_ALIAS]) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\db\migrations\executor.py", line 18, in __init__ self.loader = MigrationLoader(self.connection) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\db\migrations\loader.py", line 53, in __init__ self.build_graph() File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\db\migrations\loader.py", line 220, in build_graph self.applied_migrations = recorder.applied_migrations() File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\db\migrations\recorder.py", line 77, in applied_migrations if self.has_table(): File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\db\migrations\recorder.py", line 56, in has_table tables = self.connection.introspection.table_names(cursor) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\db\backends\base\introspection.py", line 52, in table_names return get_names(cursor) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\django\db\backends\base\introspection.py", line 47, in get_names return sorted(ti.name for ti in self.get_table_list(cursor) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\djongo\introspection.py", line 47, in get_table_list for c in cursor.db_conn.list_collection_names() File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\pymongo\database.py", line 880, in list_collection_names for result in self.list_collections(session=session, **kwargs)] File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\pymongo\database.py", line 842, in list_collections return self.__client._retryable_read( File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\pymongo\mongo_client.py", line 1514, in _retryable_read server = self._select_server( File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\pymongo\mongo_client.py", line 1346, in _select_server server = topology.select_server(server_selector) File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\pymongo\topology.py", line 244, in select_server return random.choice(self.select_servers(selector, File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\pymongo\topology.py", line 202, in select_servers server_descriptions = self._select_servers_loop( File "C:\Users\Admin\AppData\Local\Programs\Python\Python39\Lib\site-packages\pymongo\topology.py", line 218, in _select_servers_loop raise ServerSelectionTimeoutError( pymongo.errors.ServerSelectionTimeoutError: cluster0-shard-00-02.mny7y.mongodb.net:27017: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1129),cluster0-shard-00-01.mny7y.mongodb.net:27017: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1129),cluster0-shard-00-00.mny7y.mongodb.net:27017: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1129), Timeout: 30s, Topology Description: <TopologyDescription id: 6155f0c9148b07ff5851a1b3, topology_type: ReplicaSetNoPrimary, servers: [<ServerDescription ('cluster0-shard-00-00.mny7y.mongodb.net', 27017) server_type: Unknown, rtt: None, error=AutoReconnect('cluster0-shard-00-00.mny7y.mongodb.net:27017: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1129)')>, <ServerDescription ('cluster0-shard-00-01.mny7y.mongodb.net', 27017) server_type: Unknown, rtt: None, error=AutoReconnect('cluster0-shard-00-01.mny7y.mongodb.net:27017: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1129)')>, <ServerDescription ('cluster0-shard-00-02.mny7y.mongodb.net', 27017) server_type: Unknown, rtt: None, error=AutoReconnect('cluster0-shard-00-02.mny7y.mongodb.net:27017: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1129)')>]>  I am using djongo as the database engine DATABASES = { 'default': { 'ENGINE': 'djongo', 'NAME': 'DbName', 'ENFORCE_SCHEMA': False, 'CLIENT': { 'host': 'mongodb+srv://username:password@cluster0.mny7y.mongodb.net/DbName?retryWrites=true&w=majority' } } }  And following dependencies are being used in the app dj-database-url==0.5.0 Django==3.2.5 djangorestframework==3.12.4 django-cors-headers==3.7.0 gunicorn==20.1.0 psycopg2==2.9.1 pytz==2021.1 whitenoise==5.3.0 djongo==1.3.6 dnspython==2.1.0  What should be done in order to resolve this error?

Answer: This is because of a root CA Let’s Encrypt uses (and Mongo Atals uses Let's Encrypt) has expired on 2020-09-30 - namely the "IdentTrust DST Root CA X3" one. The fix is to manually install in the Windows certificate store the "ISRG Root X1" and "ISRG Root X2" root certificates, and the "Let’s Encrypt R3" intermediate one - link to their official site - https://letsencrypt.org/certificates/ Copy from the comments: download the .der field from the 1st category, download, double click and follow the wizard to install it.