404 Error Appears at Preview or Post

My client has been publishing posts every day without a problem… then today she posts one that has under 300 words in the post with 2 pictures. The last line in the post says,

Purchase <em><a href="https://www.betterworldbooks.com/little-chicken's-big-day-id-9781442414013.aspx">Little Chicken's Big Day</a></em> by Jerry and Katie Davis from <a href="https://www.betterworldbooks.com">Better World Books</a>, a Picture Book Month partner. You are "doing good" with every book you buy from Better World Books.

If we remove this part, or place it in another portion of the post, it works fine. But if we place it at the end, where we prefer it to be, we get a “You 404’d it. Gnarly, dude.” when we try and update, preview, or publish.

All our posts end similarly, just different links. I’ve tried only putting the text without the links, but we still have the same problem. I even went as far as retyping this post as a new post, thinking that perhaps my client got some kind of malformed code in there… nothing. I deactivated all plugins thinking maybe there was a bad one…but that did nothing. I’ve tried to clear cache… still get the same error.

As I’ve stated, my client posts everyday. She receives her material from writers, posts it to the blog, then schedules it’s release ahead of time. This has only happened once in 15 posts. There are 10 pages and 17 widgets as well. I’m not sure if all that is important, but I have no other ideas of how to fix this.

I’ve seen a lot of threads that have similar problems but I’ve not seen any resolve. Any help would be appreciated.

The blog is https://picturebookmonth.com and the prescheduled link to the problematic post is https://picturebookmonth.com/2011/11/why-picture-books-are-important-by-katie-davis/

It is scheduled for release 11/9/11…

Thank you.
Heidi Hafner