Before posting, I checked and re-uploaded another title that was unchanged. I got identical error behavior, so while there may be something wrong with my HTML, it seems Amazon KDP is experiencing issues.
Any advice beyond the obvious "Try later; they're probably working on it"?
I have an eBook at https://cjshayward.com/wp-content/ebooks/The%20Luddite's%20Guide%20to%20Technology.html, and I just added a chapter, "Why I'm Glad I'm Alive Now, at This Time, in This World" (ported from an online article at https://cjshayward.com/why-im-glad-im-alive-now-at-this-time-in-this-world/). KDP is giving a 500 server error (the page only shows "500" with no more specific diagnostics). I've looked at the new material, and nothing is jumping out at me as to what is wrong with this plain, old, simple HTML source.
What is corrupt in the HTML version of the eBook that Amazon KDP is choking on?