Your ROOT_URL in app.ini is "https://gitea.disconnected-by-peer.at/", it's unlikely matching the site you are visiting.
Mismatched ROOT_URL config causes wrong URL links for web UI/mail content/webhook notification/OAuth2 sign-in.
This instance is configured to run under HTTPS (by ROOT_URL config), you are accessing by HTTP. Mismatched scheme might cause problems for sign-in/sign-up.