Hi all,
while working on my themes in a 5.7.5.9 installation (on Windows 7 XAMPP 3.2.2 platform),
i.e. adding a page_theme.php file, removing and re-adding a theme, I lost access to the dashboard/login screen completely after logging out.
It showed the message: "An unexpected error occurred. Fatal error: Call to a member function getPackageHandle() on null"
C5 seemed to be unable to resolve the page /login/. The actual website continues to function correctly, only the dashboard remains unaccessible.
As my web content seemed to be lost forever, I started a new installation with 5-8.1.0 to rebuild my site. Installation went well, I added themes, pages and block content successfully. I also created a navigation block in a staple and inserted it without error on my pages.

When starting a new login session, I suddenly couldn't access this block anymore. When clicking on my navigation block under "Staple / Global areas" it showed the error:

" An unexpected error occurred. Class 'Concrete\Block\Controller' not found"

Seems to me that C5 produces inconsistent data once in a while. Any ideas on how to resolve such spontaneously broken C5 installations?

Thanks!