NY Times: After finishing Navier-Stokes, OpenAI said it had made substantial progress on a 2nd Millennium Prize problem, without identifying it or disclosing results.

And that OpenAI is deciding how to share the results.

Clay created 7 Millennium Prize Problems, Poincaré and Navier-Stokes is out of that list now, so that new problem could be any of the below,

Hodge Conjecture Birch and Swinnerton-Dyer Conjecture Riemann Hypothesis P versus NP Yang-Mills and the Mass Gap