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No breadcrumbs on any administration pages. Only the Home link shows.
This is probably not References fault, but it's one of the few things that changed between a working copy of the site and a version missing this. Indeed, the References module is enabled where i have this problem, and is magically not (haven't saved the admin/modules page since the update that added the References module as a requirement for Node Reference and User Reference) on the site that still has breadcrumbs.
Alternate suspects (another place to stick this issue!) and debugging tips welcome.