The likely cause of this is due to a previous campaign being forwarded to another person and that person choosing to opt out. When a campaign is forwarded to another person, any action that the person performs will get tracked back to your email address (or the email address that forwarded the email in the first place).
Please contact Concep Support detailing the email address required to be opted back in.
NB. If you are requesting an opt in for another person we will need an email from that person verifying their desire to be opted back in.