Hi, welcome to the Community!
Bitbucket Cloud uses id.atlassian.com for authentication before passing your session back to Bitbucket at bitbucket.org. Brave's Shield by default blocks cookies between domains. Since these are different domains, the Shield settings in Brave don't apply cleanly across the application - even if you've disabled Shield at one point, it takes a couple prods of Shield tweaking to get everything flowing well. The Optimizely and NewRelic javascript packages that you've noted should not be necessary for login. The issue is with cookies on separate domains during the authentication flow.
This isn't unique to Atlassian; I've reported this issue to Brave and noted many other sites reporting the Shield cookie blocker breaking IAM. It's notable that uBlock Origin, Disconnect, etc. don't have the same effect that Brave Shield does here. Hopefully it's sorted and "just works" soon.
In the meantime, you can follow the instructions on this post for deactivating shields in all the necessary places.
Cheers,
Daniel
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.