[UNSUBSCRIBE_URL] shortcode error

So, I've been testing some of the notifications emails with some customizations I made, and I've noticed that the links that are set with the [UNSUBSCRIBE_URL] give a "server is having problems to load" error when are clicked. Now, I noticed the url where the link redirects has the syntax wrong, displaying it like https//mydomain, WITHOUT the two dots (:). So when clicked, it sort of directs you to "https://https//mydomain". It seems like a simple solution: to correct the syntax the shortcode echoes. But, how do I do that?