Hi @zone42, please check the following issue for a more detailed description and a workaround: Logintoken are Invalidated 21.0.1 · Issue #26502 · nextcloud/server · GitHub.
The problem is that all auth tokens are renewed at each login in version 21.0.1 which causes the slow login performance.
You can apply this fix from one of the comments as a workaround until it gets resolved in a release: Logintoken are Invalidated 21.0.1 · Issue #26502 · nextcloud/server · GitHub.
Let me know if anything remains unclear!