Your ROOT_URL in app.ini is https://source.parasitstudio.de:63000/ but you are visiting https://source.parasitstudio.de/wirtz/OpenAudio_ArduinoLibrary/src/commit/3cb68260bd19dc8b1d0d4ce5fdacbbc49f1910cd/docs/bootstrap/css
You should set ROOT_URL correctly, otherwise the web may not work correctly.
Chip Audette