On firefox, trying to log in just redirects me to the help page. (possibly because it insta-signed me out)
On Chrome, the microsoft login instantly signs you out upon signing in.
Same happens in Edge.

Refreshing cache did nothing.
Either way, no success with loging in.

Looking around, this seems like a commonly reported issue.

EDIT:
Workaround mentioned in:
https://bugs.mojang.com/browse/WEB-6665

(go to "https://minecraftfeedback.zendesk.com/auth/v2/login/ " to sign in, instead of trying to log-in via the feedback site directly)

Does work. Though really, the site should just work and let me sign in normally.