BuddyPress.org

It’s not that the activation key is being “lost”. It’s that the user has already been activated, and the error being thrown by WP/BP (“provide a valid key”) is extremely unhelpful. It will be improved in BP 1.3.

Just instruct your users to log in with their username and password. They should find that their accounts are fully functional.

BTW, another plugin to help with activation is https://wordpress.org/extend/plugins/unconfirmed