Just got email from Avast with topic: "Reminder: Avast Forum Password Change".
And I have the same problem. In my.avast.com trying to change the password says: "Provided old password is not valid." How stupid is this?! I'm logged in with that password and trying to change it, like advised, Avast website tells me that it is incorrect.
Old password 30 chars long, tried new password too 30 chars long (Avast tells: "Great! This is a strong password"), same character set used.
EDIT: Ignore this, for some reason now it worked after several tries. I dunno what happened.