Regardless of roo version, external, or internal downloaded via the IDE add-ons manager, no roo shell will launch.
After installing roo, starting STS shows:
An internal error occurred during: "Initializing Spring Roo Tooling".
loader constraint violation: loader (instance of org/eclipse/osgi/internal/baseadaptor/DefaultClassLoader) previously initiated loading for a different type with name "org/osgi/framework/BundleContext"
Project specific roo settings show not installed roo instances in the roo version dropdown.