Registration submission crashes with ERR_HTTP_HEADERS_SENT after middleware fetch failure

Error

Error: Cannot append headers after they are sent to the client

Stack trace

ServerResponse.appendHeader (node:_http_outgoing:746)
  ← p / handleResponse (next/dist/esm/build/templates/app-page.js [_not-found/page]:1119)
  ← Module.N [as handler] (next/dist/esm/build/templates/app-page.js [_not-found/page]:1326)
  ← NextNodeServer.renderToResponseWithComponentsImpl (next/dist/server/base-server.js:1462)
  ← NextNodeServer.renderErrorToResponseImpl (next/dist/server/base-server.js:1808)
  ← pipe.req.req (next/dist/server/base-server.js:1660)
  ← NextNodeServer.pipeImpl (next/dist/server/base-server.js:1044)
  ← invokeRender (next/dist/server/lib/router-server.js:266)
  ← handleRequest (next/dist/server/lib/router-server.js:505)

Source (node_modules/next/dist/esm/build/templates/app-page.js:1114-1124, page=/_not-found/page)

                        }
                    } else if (typeof value === 'number') {
                        value = value.toString();
                        res.appendHeader(key, value);
                    } else {
                        res.appendHeader(key, value);
                    }
                }
            }
            // Add the cache tags header to the response if it exists and we're in
            // minimal mode while rendering a static page.

Request context

• URL: POST https://main.devguard.org/self-service/registration?flow=b4e9c21d-2488-40f3-9f1d-3e9813e58352
• Culprit route: POST /_not-found/page
• Body: registration submission with traits.name, traits.email, confirmedTerms: true (a synthetic/test account, test-user-1785750079714-...)
• Same trace ID (c617b957c0e14f679065783b5d290202) as issue 3829 — that issue is the Ory Elements client-side error ("unexpected HTML or text response") from this exact registration POST; this event is the underlying Next.js server crash causing that malformed response. A middleware fetch failed (via NextNodeServer.runMiddleware) occurred ~12ms before this error, matching the exact same pattern as issue 3418: a middleware fetch failure triggers Next's not-found/error rendering path after headers were already sent, crashing with ERR_HTTP_HEADERS_SENT and causing the response body Ory Elements receives to be an HTML error page instead of JSON.
• This confirms the root-cause chain for 3829: middleware fetch failure → ERR_HTTP_HEADERS_SENT crash → malformed/HTML 500 response → Ory Elements SDK throws "unexpected HTML or text response".

GlitchTip

• Issue: https://error-tracking.devguard.org/devguard/issues/3643
• Event ID: 019fc70069be79809d506fa045a52951
• First seen: 2026-08-03T09:42:00.862Z

Start at the middleware handling for POST /self-service/registration and trace the fetch failure through NextNodeServer.runMiddleware and the /_not-found/page rendering path. Reproduce the synthetic registration request and compare it with issues 3418 and 3829. Done means the failed middleware request produces a valid JSON error without a second response or header write, rather than an HTML error response.