I also tried to rename the child theme, and that didn’t help.
I had done a put new settings for my posts, including a lot for W3 cache and Yoast SEO, and all of the sudden no posts or page would open anymore. I found this, and dlawton’s simple solution did it. So happy! Thank you!
Thank you so much! All the answers I had found were in php, and this was just what did it, in under 10 seconds! Thank you, thank you, thank you!