BadTcpEndpointUrlInvalid error after executing set_security_string

[Max-lab-prog]

Hi,
I want to connect to an encrypted OPC UA production server. To establish a connection I use the function set_security_string. I create the necessary certificate and key as described in the example.
The serverURL, username, password, SecurityPolicy and SecurityMode are all correct.
If I now execute the script, the error shown as set_security_string is executed.
The OPC UA server is set to accept every incoming certificate automatically. However, the certificate I created is not displayed.
I can't figure out the error and when I try to connect to the Prosys Simulation Server with the same settings, it works fine.
For the creation of the certificate I have chosen
subjectAltName = URI:urn:freeopcua:client
and for the client
client.application_uri = "urn:freeopcua:client"
What am I doing wrong?
Thanks for your help

[AndreasHeine]

could you post a simplified example?

[Max-lab-prog]

import asyncio
import logging
from asyncua import Client

async def main():
    url = "opc.tcp://opcuaserver.com:48010" # add server url here
    username = "TestUser"   # set the username to access the server
    userpassword = "12345"  # set the password to access the server
    try:
        client=Client(url=url, timeout=30) # set timeout to 30 seconds
        client.set_user(username) 
        client.set_password(userpassword) 
        
        client.application_uri = "urn:freeopcua:client" # Should match in your certificate
        await client.set_security_string("Basic256Sha256,SignAndEncrypt,C:\\Users\\Name\\Desktop\\my_cert.der,C:\\Users\\Name\\Desktop\\my_private_key.pem")
        
        await client.connect() # connect to opc ua server
        print("Root children are", await client.nodes.root.get_children())
    finally:
        await client.disconnect()

if __name__ == "__main__":
    logging.basicConfig(level=logging.WARN)
    asyncio.run(main())

[Max-lab-prog]

Tomorrow I can work with the machine again, do you have an idea what I can do to prevent the error or how I can find out exactly what the problem is?