Nginx Proxy Manager: 502 Error?

I recently migrated from a PfSense router to a Ubiquiti router.

With PFSense, I was using HAProxy. I am now using Nginx Proxy Manager on a separate machine.

Nginx Proxy Manager is at The ERPNext box is at 10/1.1.21/24

I can access the ERPNext instance by its IP address, but not by domain. The domain still resolves to the right public IP.

Nginx Proxy Manager is working fine for another host (Nextcloud, in this case).