To add more info, I encountered this problem on a brand new install of Q2A 1.7, the site was sending emails correctly but clicking through the link in the sent emails resulted in the error described by Puneet Thakur above. It happened that the very first non-admin test user I created I entered with a typo in the email, so there was no way to confirm the email. I tried using the user's control panel to change their email, but it looks like you can't change an unconfirmed email address. After unsuccessfully attempting to change their email address, I created a new email account that matched the typo email address (it happened to be available), and confirmation links sent to that address repeatedly failed to work. I then logged in as administrator, use the admin control panel to change the user's email address to the correct address (which worked), after which confirmation emails for that user contained valid working links allowing the user to confirm their email successfully.
