Lets be clear, you are logging on to the .my.avast.com account and not the forums ?
It is independent of the forum as far as having posted in the forums, so there is no limit, it is how forum logon is validated, but not post related.
Why doesn't it work, what email are you clicking (I'm thinking you are clicking on the email in the avastUI main page) ?
Try logging on to your my.avast.com account using your browser and not via the avastUI.