What I’d do is to back up the child theme and then install the basic child theme they suggest, which you can find here, that new child should work then you can start implementing the customizations that you have on the old child theme into the new one and see when the errors pop again, or probably they don’t.
Why didn’t the child theme also update when the parent theme did?
Child themes shouldn’t get an update, that is the trick, you update the parent theme and the child theme keeps all the customizations.