How to fix OpenSSL.SSL.Error: [('SSL routines', '', 'certificate verify failed')] in twisted framework?

I am using MacOS ventura and pycharm and trying to establish a connection between a twisted server/client with tls following this guide: TLS server with client authentication via client certificate verification Therefore i created a self signed certificate like in this example with:

openssl req -x509 -newkey rsa:4096 -days 365 -nodes -keyout ca-key.pem -out ca-cert.pem -subj "/C=/ST=/L=/O=/OU=/CN=*.example.com/emailAddress="

openssl req -newkey rsa:4096 -nodes -keyout server-key.pem -out server-req.pem -subj "/C=/ST=/L=/O=/OU=/CN=*.example.com/emailAddress="

openssl x509 -req -in server-req.pem -days 60 -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out server-cert.pem -extfile server-ext.cnf

server-ext.cnf contains: subjectAltName=DNS:*.example.comIP:0.0.0.0

openssl verify -CAfile ca-cert.pem server-cert.pem returns OK

I merged server-key.pem and server-cert.pem in server.pem and renamed server-cert.pem in public.pem

running this for the server

\#!/usr/bin/env python

# Copyright (c) Twisted Matrix Laboratories.

# See LICENSE for details.

import sys

from twisted.internet import ssl, protocol, task, defer
from twisted.python import log
from twisted.python.modules import getModule

import echoserv

def main(reactor):
log.startLogging(sys.stdout)
certData = getModule(__name__).filePath.sibling('public.pem').getContent()
authData = getModule(__name__).filePath.sibling('server.pem').getContent()
authority = ssl.Certificate.loadPEM(certData)
certificate = ssl.PrivateCertificate.loadPEM(authData)
factory = protocol.Factory.forProtocol(echoserv.Echo)
reactor.listenSSL(8000, factory, certificate.options(authority))
return defer.Deferred()

if __name__ == '__main__':
import ssl_clientauth_server
task.react(ssl_clientauth_server.main)

and that for the client side

#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.

from twisted.internet import ssl, task, protocol, endpoints, defer
from twisted.python.modules import getModule

import echoclient

@defer.inlineCallbacks
def main(reactor):
    factory = protocol.Factory.forProtocol(echoclient.EchoClient)
    certData = getModule(__name__).filePath.sibling('public.pem').getContent()
    authData = getModule(__name__).filePath.sibling('server.pem').getContent()
    clientCertificate = ssl.PrivateCertificate.loadPEM(authData)
    authority = ssl.Certificate.loadPEM(certData)
    options = ssl.optionsForClientTLS(u'example.com', authority,
                                      clientCertificate)
    endpoint = endpoints.SSL4ClientEndpoint(reactor, 'localhost', 8000,
                                            options)
    echoClient = yield endpoint.connect(factory)

    done = defer.Deferred()
    # echoClient.connectionLost = lambda reason: done.callback(None)
    echoClient.connectionLost = lambda reason: done.callback(reason)
    yield done

if __name__ == '__main__':
    import ssl_clientauth_client
    task.react(ssl_clientauth_client.main)

the server code is working, but the client code raises an error: OpenSSL.SSL.Error: [('SSL routines', '', 'certificate verify failed')]

I tried the steps in this Answer, installed openssl via homebrew, certifi, did export SSL_CERT_FILE="$(python -m certifi)", installed service-identity but nothing helped so far.

I am expecting a connection between server and client using TLS. How to fix this?

after recreating the key/certificate with:

openssl req \
   -newkey rsa:2048 -nodes -keyout domain.key \
   -x509 -days 365 -out domain.crt

the error: OpenSSL.SSL.Error: [('SSL routines', '', 'certificate verify failed')] disappears.