Well you have me confused now, easily done.

As far as I’m aware (as an avast user) I would have thought the old avast account (at my.avast.com) should have allowed access to the avast forum. However, it rather depends on how old it really was and if you entered the login details as they were on the old my.avast.com account.

So given that you have/had forgotten that original email, you wouldn’t have been able to access that accounts access to the forum. If it did create a new my.avast.com account it would also stop duplicate information being used in another account.