As a (belated) update to this thread, I just came across another instance of this problem, after a working site was updated from php 5.x to 7.x
The error messages are not the most helpful, as it implies the paths to the framework and library files are not correct, even though they are known to be.
The problem in my case, was when the parent page and the plugin page both had the same name.
If the plugin page was loaded directly, no problem.
If the parent page did not include the plugin page, it would load fine
The problem was resolved when I renamed the plugin page.
Hope this helps somebody if you encounter this problem