Hi,
unfortunately the micropython certificate handling for MQTT has been changed last year. good description
Here is the new way to do it:

Create an SSL context

context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
context.verify_mode = ssl.CERT_REQUIRED

# Load certificates
context.load_verify_locations(cafile='/flash/certificate/ca.der')
context.load_cert_chain('/flash/certificate/cert.der', '/flash/certificate/key.der')
mqtt = MQTTClient(client_id=client, server=endpoint, port=8883, keepalive=1200, ssl=context)
print("Connecting to AWS IoT...")
mqtt.connect()
print("Done")

While the code and certificates work fine on ESP32-S3 Micropython they result in the following error on uiflow2

Traceback (most recent call last):
File "<stdin>", line 74, in <module>
File "<stdin>", line 52, in mqtt_connect
File "/flash/libs/umqttn/simple.py", line 69, in connect
File "ssl.py", line 1, in wrap_socket
ValueError:
The certificate validity has expired
I triple checked the certificate validity..
Next suspect would be an incorrect system time. Therfore I set the time as follows before attempting to initialize a mqtt connection:

sta_if = network.WLAN(network.STA_IF)
sta_if.active(True)
if sta_if.isconnected():
sta_if.disconnect()
sta_if.connect(wifi_ssid, wifi_password)
while not sta_if.isconnected():
utime.sleep(1)
print("Connecting...")
print("Connected:", sta_if.ifconfig())

print(f"time before NTP settime {time.localtime()}")
ntptime.settime()
print(f"time after NTP settime {time.localtime()}")
import machine
rtc = machine.RTC()

Set manually to a known correct UTC date/time: (year, month, day, weekday, hour, minute, second, microsecond)

rtc.datetime((2025, 3, 14, 1, 14, 0, 0, 0))
print("Manually set time:", machine.RTC().datetime())
Still I get the same error:

Connecting...
Connecting...
Connecting...
Connecting...
Connected: ('192.168.5.0', '255.255.252.0', '192.168.4.1', '192.168.4.1')
time before NTP settime (2025, 3, 14, 12, 8, 44, 5, 72)
time after NTP settime (2025, 3, 14, 12, 8, 45, 5, 72)
Manually set time: (2025, 3, 14, 5, 14, 0, 0, 67)
Connecting to AWS IoT...
MQTT Connection failed:
The certificate validity has expired

Traceback (most recent call last):
File "<stdin>", line 79, in <module>
File "<stdin>", line 57, in mqtt_connect
File "/flash/libs/umqttn/simple.py", line 69, in connect
File "ssl.py", line 1, in wrap_socket
ValueError:
The certificate validity has expired
I used the micropython/umqtt.simple/umqtt/simple.py implementation installed as module umqttn in the /flash/libs folder. It differs from the m5stack implementation (which is currently broken for certificate based MQTT due to the described changes in Micropython).

At this point I wonder why the same code (umqtt.simple, demo-code, exactly the same certificate files) work on an UMFeatherS2 MicroPython v1.24.1 but fail on MicroPython v1.24.0-dirty on 2025-03-06; M5STACK CoreS3 with ESP32S3.

I would happy for any hint on how to solve this.

Aha! When trying the following change:
context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
context.verify_mode = ssl.CERT_OPTIONAL # Instead of CERT_REQUIRED
I successfully connected!