User cannot register and add comment!

I just set up a blog here: https://thehealthyskeptic.org. I’m very new to this, so please excuse my ignorance.

A reader just sent me this message: “however it won’t allow me to login so I can leave a comment! It won’t recognize my username…just wanted to alert you, in case it was a system glitch!”

I checked my settings and I have “anyone can register” and “users must be logged in to comment” both checked. I was able to register for comments myself, but I can’t figure out why she’s having trouble.

Is it a good idea to have “users must be logged in to comment” to prevent spam? I do have the Askimet and WP-Spam Free plugins enabled. It seems like enabling the “users must be logged in…” option might discourage some people from posting and cause problems like this. But I’m new at this and not sure what to do.

I am using the “Subscribe to Comments” plugin, if that makes a difference. Any ideas?