I answered a similar question here:

• https://stackoverflow.com/a/79183434/22324694

In summary, this is currently still an issue in next-auth v5 beta. You can read up on them at:

• https://github.com/nextauthjs/next-auth/pull/10797
• https://github.com/nextauthjs/next-auth/discussions/9274

The two workarounds from the first link are:

1. @k3k8 solution (source):

   git clone -b fix-base-path https://github.com/k3k8/next-auth.git
   cd next-auth
   corepack enable pnpm
   pnpm install
   pnpm build
   cd packages/next-auth/
   pnpm pack
   

   This will generate a package file named something like next-auth-5.0.0-beta.20.tgz.

   Move this file to your project directory and install it by updating your package.json:

   {
     "dependencies": {
       "next-auth": "./next-auth-5.0.0-beta.20.tgz"
     }
   }
   

   Then run pnpm install

2. @ThangHuuVu's solution (source):

   // app/api/auth/[...nextauth]/route.ts
   import { handlers } from "auth";
   import { NextRequest } from "next/server";
   
   const reqWithBasePath = (req: NextRequest, basePath = "/authjs") => {
     const baseUrl = new URL(
       (req.headers.get("x-forwarded-proto") ?? "https") +
         "://" +
         (req.headers.get("x-forwarded-host") ?? req.nextUrl.host)
     );
     const urlWithBasePath = new URL(
       `${basePath}${req.nextUrl.pathname}${req.nextUrl.search}`,
       baseUrl
     );
     return new NextRequest(urlWithBasePath.toString(), req);
   };
   
   export const GET = (req: NextRequest) => {
     const nextReq = reqWithBasePath(req);
     return handlers.GET(nextReq);
   };
   
   export const POST = (req: NextRequest) => {
     const nextReq = reqWithBasePath(req);
     return handlers.POST(nextReq);
   };
   

   https://github.com/ThangHuuVu/next-auth-behind-proxy/blob/cde4368b46fbda57157d62eaf1b93b258f5fadde/app/api/auth/%5B...nextauth%5D/route.ts