Letsencrypt is shutting down their V1 API for registrations tomorrow. (https://community.letsencrypt.org/t/end-of-life-plan-for-acmev1/88430). Unfortunately, we shipped 19.09 without V2 API support. This means any new certificate creations with the current NixOS modules will fail from tomorrow own.
Luckily, we just backported a fix (https://github.com/NixOS/nixpkgs/pull/71953). Please update your channel avoid any service disruptions.
If you have any questions about this topic, let me know!