Matrix integration matrix.org only? any frei.chat users?

Hi there,
I am struggling to get the Matrix integration in core 2024.2.1 running.

My configuration looks like:

matrix:
  homeserver: https://frei.chat
  username: "@[xxx]:frei.chat"
  password: !secret [xxxpw]
  rooms:
    - "#[xxx.yyy]:frei.chat"

notify:
  - name: matrix_notify
    platform: matrix
    default_room: "#[xxx.yyy]:frei.chat"

“[xxx]” is a placeholder. There are no square brackets in my names but yes, my room name contains a dot.

All what I get in the logs is the message:

Logger: nio.responses
Source: components/matrix/__init__.py:395
First occurred: 3:17:54 PM (1 occurrences)
Last logged: 3:17:54 PM

Error validating response: 'user_id' is a required property

I had a look in core/homeassistant/components/matrix/__init__.py at 6aae44dbb3efd8f323ace5075b59e703cef24fe3 · home-assistant/core · GitHub but cannot really get a clue.

Probably 'user_id' is a required property is a response of frei.chat which cannot be parsed? But frei.chat is using standard Matrix server.

Any hints and tips welcome.