Unable to opt in to Cataclysm Beta

Unable to opt-in to Cataclysm Beta. (I would include the links but apparently that’s not allowed).

When using link at the wow classic beta-opt in I get redirected to an error page, giving me: Battle.net - OAuth 2.0 Error. 400 Invalid grant type or callback URL is not valid.

At the wow.classic.blizzard webpage I am unable to log in to my account via the drop down at the top right - I get the same response.

Any help would be appreciated.

I’ve been having the same issue for a few days now, Are there any fixes?