"Page Not Found" on specific child pages

Have you tried disabling all plugins?

Happens across multiple themes, and with all the the plugins disabled.

Try setting the permalinks again. Settings -> permalnks to Post Name.

First set it somethig else and then again set it to post. It will solve your problem.

That didnt work, but you were close!

Turns out I had News set as a the “Category Base”. I removed that, and now it works perfectly!