@Sólveig Thanks for reporting this

Error 431 means the browser is sending more cookie data than the server accepts. This happens to long-time DAREBEE users: before we moved tracking to local storage, every completed workout was stored as a cookie, and after years of training those pile up.
We've just raised the server's limit, so could you try loading the site again first?
If you can get in, please go to your Dashboard > Settings and tap Clean up old cookie data at the bottom. That removes only the old cookie-based tracking and leaves your current progress alone. We'd still suggest doing a Backup from the dashboard first, just for peace of mind.
If the site still won't load at all, let us know and we'll figure out another route.