Answer
This has been resolved by dropping, recreating, and rebuilding the relevant security-related indexes — an RMAN-based database duplication can leave certain indexes in an invalidated or inconsistent state relative to the cloned data, similar to how Search Domain indexes can be invalidated by cloning, and security-information retrieval failing at login is one manifestation of that same category of post-clone index problem. Identifying and rebuilding the affected indexes (rather than assuming a permissions or configuration issue) resolved the login error.