Thank you for reaching out to the Community! I'm sorry to hear some of your contacts are having issues registering for your event. I understand how important it is for your contacts to easily register.
We definitely want to take a look into this issue for you. Please email us at social_support(at)constantcontact(dot)com with your username, the name of your event campaign, a name of a contact who has had issues registering, error messages your contacts are receiving (if any), and a reference to this post and we will investigate further!