Single User Comment Pop Up

I’ve been trying to work with a commenter on my site regarding this issue but can’t seem to find a workaround even with different browsers. Most recently, he wrote me the following,

“I’m still having problems posting comments. The pop-up is still
coming up and asking for a confirmation password that I’m typing in
correctly and then it all goes haywire and doesn’t allow me to comment. I’ve
tried it on a couple of different computers, but it is the same problems
over again.”

When I, myself, post comments, there is no pop up, so I am confused why this is in the first place.