I copied the link you shared and tried to replicate the error, but I could not. The 'Read More' block accepts the URL as well as just creating standard hyperlinked text. Are there any other details you could share that might tip us off as to what might be causing the error? Maybe share a screenshot of what you're seeing in your account?
Looking to level up your email template designs? Share your email campaigns in Template Feedback & Inspiration and receive valuable advice from Constant Contact experts and industry peers!