Python Flask server getting 'code 400' errors (POST request sent from Telegram-webhook)

You need to have SSL enabled for this to work. Telegram is trying to initiate an SSL session with your server, but you don't have SSL enabled, so you are seeing the bad request.

ssl_context='adhoc' may work for a test app, but I also have a hunch that telegram requires a VALID SSL certificate, not just an ad-hoc (or self-signed one). Notice the lock to the left of the URL in the video and the lack of a security warning that would be present with an invalid or self-signed certificate.

To make sure SSL is working set the ssl_context to adhoc, start the app, and browse to https://myprivatedomain.com:8443/index. If you can browse to it, then Telegram will also be able to browse to it, after getting a valid certificate, of course.

Next, to get a valid (and free) SSL certificate you can use LetsEncrypt.

Once you have a valid SSL certificate and key file you can pass the ssl_context argument to app.run with a tuple of the path to the certificate file and the path to the key file ("/path/to/fullchain.pem", "/path/to/privkey.pem")

Your full run function should look like this

app.run(debug=True, host='0.0.0.0', port=8443, ssl_context=("/path/to/fullchain.pem", "/path/to/privkey.pem"))

Alternatively you can use Apache or Nginx to secure your site with SSL, and reverse proxy to your bot. Those options would normally be used in a final product, so I understand if you don't want to get into the weeds with them right now, but it is good practice regardless.

Hopefully that helps.