If you’re running pi-hole behind a reverse proxy and upgrading to pi-hole 6 you are not able to login. You need to make one change in your reverse proxy.

I use nginx proxy manager and you need to expose the /api.

You should have already added /admin to your reverse proxy. You need to click the add location button at the top and then add /api to port 80 on the IP address of your pi-hole.

Credit to jclendineng in this thread: How to configure Nginx Proxy Manager correctly to use v6? - Help / Community Help - Pi-hole Userspace reported the same problem and then, like a hero, came back to report their fix: Edit: you have to expose /api