Error occurred (InvalidClientTokenId) when calling the AssumeRole operation: The security token included in the request is invalid

First I'd like to point this is NOT the same problem as this similar question. As I have looked through all the answers and none could actually help in my case.

I have a Python script that must be executed inside containers (we're going to ship it as an encapsulated solution to run on-premises for some customers) in which I have to send an email using boto3.

here is the code for it:

from os import stat
import boto3
from botocore.config import Config
import botocore.exceptions

class EmailHandler:
    def __init__(self, business_rules, integrations, credentials, event):
        self.business_rules = business_rules
        self.integrations = integrations
        self.credentials = credentials
        self.event = event 


    def send_message(self, message):
        region_name = 'us-east-1'
        role_arn = self.credentials['role_arn']

        my_config = Config(
            region_name = region_name,
            signature_version = 'v4',
            retries = {
                'max_attempts': 10,
                'mode': 'standard'
            },
        )
        
        sts = boto3.client('sts',
        aws_access_key_id=self.credentials['AWS_ACCESS_KEY_ID'],
        aws_secret_access_key=self.credentials['AWS_ACCESS_KEY_ID'])

        response = sts.assume_role(
        RoleArn=role_arn,
        RoleSessionName = 'SES_operation'
        )

        tempAccessKeyId = response['Credentials']['AccessKeyId']
        tempSecretAccessKey = response['Credentials']['SecretAccessKey']
        tempSessionToken = response['Credentials']['SessionToken']

        client = boto3.client('ses', 
        aws_access_key_id = tempAccessKeyId,
        aws_secret_access_key = tempSecretAccessKey,
        aws_session_token = tempSessionToken,
        config=my_config)
        
        subject = self.event['alert_message']
        to_email = self.credentials['email_receivers']

        status = 900
        state = 'failure'
        data = {'message': ''}
        
        try:
            response = client.send_email(
                Source= self.credentials['source_email'],
                Destination= {
                    'ToAddresses': [
                        to_email,
                    ]
                },
                Message={
                    'Subject': {
                        'Data': subject,
                    },
                    'Body': {
                        'Text': {
                            'Data': message,
                        },
                        'Html': {
                            'Data': '<p>'+ message +'</p>'
                        }
                    }
                },
                SourceArn = self.credentials['source_identity'],
            )
            state = 'success'
            status = 200
            data = {'message': response.MessageId }

        except botocore.exceptions.ClientError as error:
            data = {'message': error.response['Error']['Message']}
            raise error
            
        return(status, state, data)

As you can see, I am trying to assume role so I can use the session token provided to create a client for SES.

The problem is that I am getting the following error whenever this handler is executed:

An error occurred (InvalidClientTokenId) when calling the AssumeRole operation: The security token included in the request is invalid.

But I'm not providing any tokens since my goal is exactly to get it using this method call. Am I missing something?

PS: I can't use config file as per you can see in the code, the credentials are passed from an outside source.