This is the ticket where we will continue working on this issue. I have set your next answer private so you can share the access credentials to the staging site in a secure way.
Hi there,
Thank you for creating the staging site and sharing the credentials
I deactivated all plugins and activated a default WordPress theme, and then 28 pending updates appeared. After updating all outdated plugins the fatal error disappeared.
Could you confirm that the fatal error is gone?
Note: you will see warnings but these are not fatal errors 🙂