If you are able to login to Account Management but have found yourself being sent back to Account Management every time you try to access the Beta Forums via the e-mail link, try following these instructions (I had the same problem): Log into Account Management -> Go to Services and Options and select Service -> Select FFXIV -> Under Active Service Accounts you will see two buttons, Options and Renew (or similar) Service -> Select Renew Service -> Select Reactivate Account (it looks like you are trying to reinstate your old FFXIV account). Once you do this, it will make your registered Beta account active and then you may proceed to the Beta Forums. I know the posters telling you to read the instructions may not have dealt with this issue so wouldn't have any idea what it is you are trying to resolve. This is what worked for me - Hope it helps!
Source:
http://eu.square-enix.com/en/games/final-fantasy-xiv#comment-44047#comment-44003