That error is thrown when a subscription is attempted to be loaded for a specific plan, but the subscription doesn't exist. That can happen for example when an IPN from the payment gateway comes through for a subscription that no longer exists.
This appears to be triggered when a front-end user tries to add one of our free subscriptions (plan8), without a paid one (but I'm not 100% sure of that)
Is your free plan a free lifetime plan? If so try enabling subscriptions for free lifetime plans within CBSubs > Settings > Global.