Same here on 2 accounts, both having 2fa through a HW key(yubikey; though passkeys have the same behavior). At some point today(few hours ago) both my desktop and phone got reditected to x.com/account/access, where the loop started.

Frustratingly enough, i had already done the "re-enrollment" a long time ago(basically when they announced it was mandatory), but it seems like that was pointless(hopefully not).

I saw some prompts about birdhouse, re-did the enrollment, and badly enough (I think i dug my own hole with this one) it asked to remove the other 2FA option (SMS), to which I clicked yes.

This might sound bad but I sincerely hope X fixes it somehow, and all the keys enrolled/re-(re-[etc])-enrolled are not lost, especially those that were not added today. It might be a good idea (in practice, bad for security) to disable this new "https://x.com/account/access?flow=two-factor-security-key-po..." garbage fully, as I don't see myself contacting X support anytime soon(for obvious reasons).