I had exactly same problem and solved by disabling One Signal Plugin, thanks! This problem occurred right after updating onesignal plugin to latest version. Only hooks were affected, I was able to update post and pages.
Yes, the same happens to me. I have 3 sites using GeneratePress Theme. I have used OneSignal in 2 sites and after updating the OneSignal to the latest version this issue occurs.
I think we need to contact OneSignal Team.