Generalized Chen's conjecture on biharmonic submanifolds in non-positively curved manifolds

Let $M$ be a biharmonic submanifold of a non-positively curved Riemannian manifold. Generalized Chen's conjecture. The submanifold $M$ is minimal. Ou and Tang constructed counterexamples to this conjecture, so the claim is refuted in general. It remains open for biharmonic submanifolds in hyperbolic spaces in the cases described in the source.