Self-signed Certificate error on Dedicated Server when using handleI18nRouting middleware

[AndreyPerunov]

Hello community,

I am getting an Internal Server Error when I navigate to / or /en to the dev version of application.

Error log:

Failed to proxy https://dev.vexen.eu/en [Error: self-signed certificate; if the root CA is installed locally, try running Node.js with --use-system-ca] {
  code: 'DEPTH_ZERO_SELF_SIGNED_CERT'
}
[Error: self-signed certificate; if the root CA is installed locally, try running Node.js with --use-system-ca] {
  code: 'DEPTH_ZERO_SELF_SIGNED_CERT'
}

Environment:

• next.js v15.2.4
• nginx as reverse proxy
• Application running in Docker
• Self-signed SSL certificate (server is in office LAN)
• Using next-auth v5.0.0-beta.29 for authentication
• Using next-intl v3.26.4 for internationalization

Problem isolation:

The app works fine until I add handleI18nRouting from next-intl middleware.

• If I move handleI18nRouting(req) to the top of the middleware chain, I immediately get the error.
• If I remove it, everything works as expected.

My middleware.ts:

import createMiddleware from "next-intl/middleware"
import { routing } from "@/i18n/routing"
import { apiAuthPrefix, authRoutes, DEFAULT_LOGIN_REDIRECT, publicAuthRoutes } from "./routes"
import authConfig from "@/auth.config"
import NextAuth from "next-auth"

const handleI18nRouting = createMiddleware(routing)

const { auth } = NextAuth(authConfig)

export default auth(req => {
  return handleI18nRouting(req)
  
  // everything is ignored next

  ...

  return null
})

export const config = {
  matcher: ["/((?!.+\\.[\\w]+$|_next).*)", "/", "/(api|trpc)(.*)"]
}

Assumptions

First, I thought that the problem is with redirect, so I do this:

import createMiddleware from "next-intl/middleware"
import { routing } from "@/i18n/routing"
import { apiAuthPrefix, authRoutes, DEFAULT_LOGIN_REDIRECT, publicAuthRoutes } from "./routes"
import authConfig from "@/auth.config"
import NextAuth from "next-auth"

const handleI18nRouting = createMiddleware(routing)

const { auth } = NextAuth(authConfig)

export default auth(req => {
  if (req.nextUrl.pathname === "/") {
    const url = new URL("/en", req.url)
    return NextResponse.redirect(url.toString())
  }
  
  // everything is ignored next

  ...

  return null
})

export const config = {
  matcher: ["/((?!.+\\.[\\w]+$|_next).*)", "/", "/(api|trpc)(.*)"]
}

Result: I didn't get an error.

Question:

What could be causing this issue? Is there a known problem with combining next-intl middleware and auth.js when using a self-signed certificate in development?

Any guidance would be appreciated!