Issues with sending messages to Telegram Alert channel

Hi Experts,

I am running a Grafana 8.2.1 server (self hosted) with an InfluxDB. We routinely send alert messages via Email Alert channel. Now my team would like to use the Telegram Alert channel to reduce service response times.
For that I configured a Telegram bot and a Telegram group chat. I do have the API token and the chat_id. I am able to send messages to the group using Telegram API „/sendMessage“ - via a browser and via cURL.
However, when entering the information I have in the Telegram Alert channel confit in the browser, I get „Failed to send alert“ messages and nothing happens.
What could be wrong? Maybe server configs?