Comment submission generates 404 error, or nothing

When permalinks are set to anything but default, my site generates a 404 error upon submission of a comment. The URL looks this this:
https://www.markwhittaker.com/comments-broken/comment-page-/#comment-

When permalinks are set to default, the site returns this page upon submission: https://www.markwhittaker.com/?p=593#comment-

and no comment appears. Sometimes I get an e-mail notifying me about the comment, and when I do, all of the info fields are blank except for the title of the post.

Thanks for any advice you can provide!