Blank Confirmation Email for New Subscribers

When visitors to my site click on the comments link on a post and they’re not logged in, they are given a chance to subscribe. Many have done so but I just found out that the email they receive that is supposed to tell them their login and password is going out with zero content in it. That is, they get the email but there is no text in it at all.

I have searched until my eyes bleed and I cannot for the life of me figure out where the Register link is even presented, let alone handled. I’m using the brownline theme and WP standard default registration behavior without plugin assistance.

How do I go about solving this problem?