I am having the same issue on Windows 11, in Firefox and Chrome, even after clearing cookies. Sign in, enter the 2-factor authentication that Apple pops up on my phone, get the sign in dialog a few seconds later.
In the developer logs I see a request to https://api.apple-cloudkit.com/database/1/iCloud.net.shinyfrog.bear/production/public/users/caller that got a 421 response.
The request’s querystring is ?ckWebAuthToken=null&ckAPIToken=[some valid-looking token]
@TehShrike@jujoje@rodolform@bib we’re looking into this but there have been no changes recently on the Bear login logic. Are you using ADP by any chance? That caused issues in the past (but it was apparently solved by Apple recently).
I just double-checked, and Advanced Data Protection is off. Right now, if I try to log in, after about two seconds Bear Web disconnects my account, and the login page appears on the screen again.
@TehShrike@jujoje@rodolform@bib could you check if you’re still experiencing this issue? We found (and fixed) a regression in the way assets of conflicted notes were loaded that could justify the issue you experienced (i.e. the session being terminated and the log in screen showing up again).