I'm having the exact same issue as this closed post. I've followed all the steps from both user and the mod. On a clean OS install with only pihole, it reads "Active" and I can save any changes I make options. After installing a few other apts, it reads "unknown". I've tried serving the site with lighttpd and nginx. Additionally, I have used:
You're using nginx which isn't a supported web interface.
[80] is in use by nginx (https://docs.pi-hole.net/main/prerequisites/#ports)
[80] is in use by nginx (https://docs.pi-hole.net/main/prerequisites/#ports)