Why does accessing a protected static inner class X of a superclass Y fail with "error: class X cannot be accessed in object Y"?

This is a known limitation of Scala: there is no notion of 'static' members in Scala.  Instead, Scala treats static members of class Y as members of the singleton object Y (the companion object of class Y). When inheriting from this class, one can access only protected members of class Y but cannot access protected members of object Y.

There's no way Scala can simulate static protected without impairing the integrity of Scala's object model in a fundamental way, so this is not going to change. To work around this limitation, one has to create an implementation of the enclosing class with Java code which encapsulates all accesses to the protected static inner class.

See ticket #1806 [1] for more information and a concrete example of the limitation and its workaround.