I haven’t heard of any announcement so it’s probably an error or expired certificate.
Based on the thread below, it looks to be specific to Xbox only (which might explain why you are able to login @GTvsForzaI've been able to play fine the last few days. Then again, I never did multiplayer.
Just as an update on this, it is still impossible to get online on Grid Legends on Xbox.I play on PS, so makes sense.