Forward auth with NPM gives Error 500 in firefox, wrong proxy_set_header config/documentation may be the problem

[MrSliff]

Describe the bug
Authentik forward auth with internal outpost and nginx(-proxy-manager) gives me an Error 500 when i try to open my protected sites in Firefox. MS Edge works without errors.

To Reproduce
Steps to reproduce the behavior:

1. Set up forward-auth as desribed in the tutorials
2. Add the NginxProxyManager config to the "Advanced" Tab, edit proxy_pass in /outpost.goauthentik.io accordingly
3. Try to open website -> keeps loading and returns Error 500 after some time

Expected behavior
Website redirects to Authentik and start the authentication process.

Version and Deployment (please complete the following information):

• authentik version: [e.g. 2024-12-3]
• Deployment: docker-compose

Additional context

It seems the internal and external documentation is still incorrect. According to #2594, the documentation for NPM inside of authentik and also on the authentik homepage tutorials may be still incorrect.

Both show proxy_set_header Host $host; for the /outpost.goauthentik.io block.

I set the header to proxy_set_header X-Forwarded-Host $host; and now it is working without errors.