リバースプロキシ下で動作するように修正

nextauthjs/next-auth#10928 (comment)
とかに対処法がある

src/app/api/auth/[...nextauth]/route.ts

@@ -1,4 +1,29 @@
 import { handlers } from "@/auth" // Referring to the auth.ts we just created
-export const { GET, POST } = handlers
-export const runtime = "edge" // optional
+import { NextRequest } from 'next/server';
+//export const { GET, POST } = handlers
+//export const runtime = "edge" // optional
 
+const basePath = '/sekirei-todo';
+
+const toForwardedRequest = (req: NextRequest): NextRequest => {
+  const forwardedHost = req.headers.get('x-forwarded-host');
+  const forwardedProto = req.headers.get('x-forwarded-proto');
+  if (forwardedHost && forwardedProto) {
+    const forwardedUrl = 
+      `${forwardedProto}://${forwardedHost}${basePath}${req.nextUrl.pathname}?${req.nextUrl.searchParams.toString()}`;
+    const newReq = new NextRequest(forwardedUrl, {
+      headers: req.headers,
+      method: req.method,
+      body: req.body,
+    });
+    return newReq;
+  } else {
+    return req;
+  }
+}
+
+export const GET: (req: NextRequest) => Promise<Response> = 
+  (req) => handlers.GET(toForwardedRequest(req));
+
+export const POST: (req: NextRequest) => Promise<Response> = 
+  (req) => handlers.POST(toForwardedRequest(req));