I assume it’s only the child-theme which encounters this error – and therefore it is probably a small error – but not seeing any errors in the logs makes it harder to determine exactly where the error is.
First of all, I would suggest you turn on your WP_DEBUG in the wp-config.php, maybe there’s an error you’re missing. You can find more information on the WP_DEBUG define here.
If no errors are shown, please check your console for any errors (Right click > Inspect > console tab). I’m certain there’s an error either by activating WP_DEBUG or checking the console.
I have experienced this error before, and whenever I was saving a post, there was an AJAX-error occuring. I fixed this by starting over with the child-theme, as I must have forgot something.