If it was working as of a few months ago and isn't now, then it's probably due the removal of the old ACMEv1 endpoint.
It had been only allowing renewals for existing names for a long time to let people upgrade, but has finally gone away for good.
If that's the case, the only solution is to upgrade your client to a system that's uses the updated endpoint. If it's integrated with your software, you probably just need to upgrade your software to a version from within the past few years.