/auth/callback error when using custom domain

[RYFN]

Checklist

• The issue can be reproduced in the nextjs-auth0 sample app (or N/A).
• I have looked into the Readme, Examples, and FAQ and have not found a suitable solution or answer.
• I have looked into the API documentation and have not found a suitable solution or answer.
• I have searched the issues and have not found a suitable solution or answer.
• I have searched the Auth0 Community forums and have not found a suitable solution or answer.
• I agree to the terms within the Auth0 Code of Conduct.

Description

(nextjs-auth0 sample app is v3, so reproduction not possible)

Upgrading from v3, using an existing App, I see a "An error occured while trying to exchange the authorization code." error in the /auth/callback route.

When I use the regular *.eu.auth0.com from my AUTH0_DOMAIN config it works as expected.

When using my custom domain instead, I see the above error.

full stack trace;

'OperationProcessingError: "response" is not a conform Token Endpoint response (unexpected HTTP status code)\n    at OPE (webpack-internal:///(middleware)/./node_modules/.pnpm/oauth4webapi@3.3.1/node_modules/oauth4webapi/build/index.js:183:12)\n    at checkOAuthBodyError (webpack-internal:///(middleware)/./node_modules/.pnpm/oauth4webapi@3.3.1/node_modules/oauth4webapi/build/index.js:951:15)\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async processGenericAccessTokenResponse (webpack-internal:///(middleware)/./node_modules/.pnpm/oauth4webapi@3.3.1/node_modules/oauth4webapi/build/index.js:1234:5)\n    at async processAuthorizationCodeOpenIDResponse (webpack-internal:///(middleware)/./node_modules/.pnpm/oauth4webapi@3.3.1/node_modules/oauth4webapi/build/index.js:1445:20)'

thrown in /nextjs-auth0/dist/server/auth-client.js, line 262

This custom domain is not new, it is in use for several other apps, and works as expected with the v3 SDK

Reproduction

1. simple src/lib/auth0.ts

import { Auth0Client } from "@auth0/nextjs-auth0/server";
import { NextRequest, NextResponse } from "next/server";

/**
 * Create a bare-minimum Auth0 client using environment variables
 */
export const auth0 = new Auth0Client();

2. simple root src/app/page.tsx

import { auth0 } from "@/lib/auth0";
import Link from "next/link";

export default async function Home() {
const session = await auth0.getSession();
... do something with session, login button etc
}

3. barebones middleware

import type { NextRequest } from "next/server";
import { NextResponse } from "next/server";
import { auth0 } from "./lib/auth0";

export async function middleware(request: NextRequest) {
  try {
    // Let Auth0 handle authentication routes
    return await auth0.middleware(request);
  } catch (error) {
    console.error("Auth0 middleware error:", error);
  }
}

export const config = {
  matcher: [
    /*
     * Match all request paths except for the ones starting with:
     * - _next/static (static files)
     * - _next/image (image optimization files)
     * - favicon.ico, sitemap.xml, robots.txt (metadata files)
     */
    "/((?!_next/static|_next/image|favicon.ico|sitemap.xml|robots.txt).*)"
  ]
};

my .env.local looks like;

AUTH0_SECRET='...'
APP_BASE_URL='http://localhost:9000'
AUTH0_DOMAIN='my-domain.auth0.com'      OR     'my-custom-domain.com' 
AUTH0_CLIENT_ID='...'
AUTH0_CLIENT_SECRET='...'
API_BASE_URL=http://localhost:3000
AUTH0_AUDIENCE=https://other-server

Additional context

My auth0 logs display "Success Login" for logins for both the custom domain, an the .auth0 domain configs, and no other errors.

nextjs-auth0 version

4.0.3

Next.js version

14.1.4

Node.js version

21.5.0