The gradebook entity provider is not initializing on 2.8.x instances of Sakai. It is appearing properly in 2.8.1.
This is being caused by the bean definition for the entity provider being missing from gradebook/app/sakai-tool/src/webapp/WEB-INF/applicationContext.xml. This definition has been missing in all 2.8 versions, however it was working in the past because during build-time the applicationContext.xml at gradebook/app/ui/src/webapp/WEB-INF was actually being used instead. This can be confirmed by building a 2.8.1 and 2.8.x gradebook separately and then inspecting the applicationContext.xml files in the src/ and target/ locations for gradebook/app/sakai-tool and gradebook/app/ui.