diff --git a/messages/de.json b/messages/de.json
index 4dcc47c51..737f6dce7 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -529,6 +529,7 @@
     "noAccount": "Noch kein Konto?",
     "hasAccount": "Bereits ein Konto?",
     "loginFailed": "Anmeldung fehlgeschlagen",
+    "signOutFailed": "Abmelden hat nicht geklappt. Du bist weiterhin angemeldet.",
     "registerFailed": "Registrierung fehlgeschlagen",
     "invitedBanner": "Du wurdest eingeladen. Erstelle dein Konto, auch wenn die Registrierung sonst geschlossen ist.",
     "alreadySignedIn": {
diff --git a/messages/en.json b/messages/en.json
index 6a5c05ac9..8877c35d5 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -529,6 +529,7 @@
     "noAccount": "Don't have an account?",
     "hasAccount": "Already have an account?",
     "loginFailed": "Login failed",
+    "signOutFailed": "Could not sign out. You are still signed in.",
     "registerFailed": "Registration failed",
     "invitedBanner": "You have been invited. Create your account even while registration is otherwise closed.",
     "alreadySignedIn": {
diff --git a/messages/es.json b/messages/es.json
index a2b612bad..ad30b5020 100644
--- a/messages/es.json
+++ b/messages/es.json
@@ -529,6 +529,7 @@
     "noAccount": "¿Aún no tienes cuenta?",
     "hasAccount": "¿Ya tienes cuenta?",
     "loginFailed": "No se ha podido iniciar sesión",
+    "signOutFailed": "No se pudo cerrar la sesión. Tu sesión sigue abierta.",
     "registerFailed": "No se ha podido completar el registro",
     "invitedBanner": "Te han invitado: crea tu cuenta aunque el registro esté cerrado.",
     "alreadySignedIn": {
diff --git a/messages/fr.json b/messages/fr.json
index af97df036..ed15c1527 100644
--- a/messages/fr.json
+++ b/messages/fr.json
@@ -529,6 +529,7 @@
     "noAccount": "Pas encore de compte ?",
     "hasAccount": "Déjà un compte ?",
     "loginFailed": "Échec de la connexion",
+    "signOutFailed": "La déconnexion a échoué. Votre session est toujours ouverte.",
     "registerFailed": "Échec de l’inscription",
     "invitedBanner": "Vous avez été invité. Créez votre compte même si les inscriptions sont fermées.",
     "alreadySignedIn": {
diff --git a/messages/it.json b/messages/it.json
index fed51449c..bb602f60a 100644
--- a/messages/it.json
+++ b/messages/it.json
@@ -529,6 +529,7 @@
     "noAccount": "Non hai ancora un account?",
     "hasAccount": "Hai già un account?",
     "loginFailed": "Accesso non riuscito",
+    "signOutFailed": "Non è stato possibile uscire. La sessione è ancora attiva.",
     "registerFailed": "Registrazione non riuscita",
     "invitedBanner": "Sei stato invitato: crea il tuo account anche se la registrazione è chiusa.",
     "alreadySignedIn": {
diff --git a/messages/ko.json b/messages/ko.json
index 77eee65e5..a7baf05aa 100644
--- a/messages/ko.json
+++ b/messages/ko.json
@@ -529,6 +529,7 @@
     "noAccount": "계정이 없나요?",
     "hasAccount": "이미 계정이 있나요?",
     "loginFailed": "로그인 실패",
+    "signOutFailed": "로그아웃하지 못했어요. 아직 로그인되어 있어요.",
     "registerFailed": "회원가입 실패",
     "invitedBanner": "초대를 받았어요. 회원가입이 닫혀 있어도 계정을 만들 수 있어요.",
     "alreadySignedIn": {
diff --git a/messages/nl.json b/messages/nl.json
index 1df8e98bf..67807173c 100644
--- a/messages/nl.json
+++ b/messages/nl.json
@@ -529,6 +529,7 @@
     "noAccount": "Nog geen account?",
     "hasAccount": "Heb je al een account?",
     "loginFailed": "Inloggen mislukt",
+    "signOutFailed": "Uitloggen is mislukt. Je bent nog steeds ingelogd.",
     "registerFailed": "Registratie mislukt",
     "invitedBanner": "Je bent uitgenodigd. Maak je account aan, ook al is registratie verder gesloten.",
     "alreadySignedIn": {
diff --git a/messages/pl.json b/messages/pl.json
index fa9457d77..6bc8109e9 100644
--- a/messages/pl.json
+++ b/messages/pl.json
@@ -529,6 +529,7 @@
     "noAccount": "Nie masz jeszcze konta?",
     "hasAccount": "Masz już konto?",
     "loginFailed": "Logowanie nie powiodło się",
+    "signOutFailed": "Nie udało się wylogować. Sesja jest nadal aktywna.",
     "registerFailed": "Rejestracja nie powiodła się",
     "invitedBanner": "Otrzymano zaproszenie. Utwórz konto, nawet jeśli rejestracja jest zamknięta.",
     "alreadySignedIn": {
diff --git a/messages/pt-BR.json b/messages/pt-BR.json
index 6abf9a204..5efe0f404 100644
--- a/messages/pt-BR.json
+++ b/messages/pt-BR.json
@@ -529,6 +529,7 @@
     "noAccount": "Não tem uma conta?",
     "hasAccount": "Já tem uma conta?",
     "loginFailed": "Falha no login",
+    "signOutFailed": "Não foi possível sair. Sua sessão continua ativa.",
     "registerFailed": "Falha no cadastro",
     "invitedBanner": "Você foi convidado. Crie sua conta mesmo que o cadastro esteja fechado.",
     "alreadySignedIn": {
diff --git a/messages/ru.json b/messages/ru.json
index 159cf4fab..412c87fa9 100644
--- a/messages/ru.json
+++ b/messages/ru.json
@@ -529,6 +529,7 @@
     "noAccount": "Нет аккаунта?",
     "hasAccount": "Уже есть аккаунт?",
     "loginFailed": "Не удалось войти",
+    "signOutFailed": "Не удалось выйти. Вы по-прежнему в системе.",
     "registerFailed": "Не удалось зарегистрироваться",
     "invitedBanner": "Вас пригласили. Создайте аккаунт, даже если регистрация в остальном закрыта.",
     "alreadySignedIn": {
diff --git a/src/app/auth/register/__tests__/page.browser.test.tsx b/src/app/auth/register/__tests__/page.browser.test.tsx
index 0182022db..be17d6e25 100644
--- a/src/app/auth/register/__tests__/page.browser.test.tsx
+++ b/src/app/auth/register/__tests__/page.browser.test.tsx
@@ -46,3 +46,27 @@ describe("registration page", () => {
     );
   });
 });
+
+describe("registration page, already signed in", () => {
+  it("keeps the page and says so when the server refuses the sign-out", async () => {
+    const api = stubFetch().on("POST", "/api/auth/logout", {
+      status: 403,
+      error: "Demo mode: modifications are disabled",
+      meta: { demo: true, errorCode: "demo.readOnly" },
+    });
+    window.history.replaceState(null, "", "/auth/register?invite=hlv_x");
+    const { screen, queryClient } = await renderWithApp(<RegisterPage />, {
+      user: { username: "signed-in" },
+    });
+
+    await screen.getByTestId("register-sign-out").click();
+
+    await expect
+      .element(screen.getByText("Could not sign out. You are still signed in."))
+      .toBeVisible();
+    expect(api.calls("POST", "/api/auth/logout")).toHaveLength(1);
+    // Nothing was wiped and nobody was sent anywhere: the session is alive.
+    expect(queryClient.getQueryData(queryKeys.authMe())).toBeDefined();
+    expect(window.location.search).toBe("?invite=hlv_x");
+  });
+});
diff --git a/src/app/auth/register/page.tsx b/src/app/auth/register/page.tsx
index d88ba1c32..592917c88 100644
--- a/src/app/auth/register/page.tsx
+++ b/src/app/auth/register/page.tsx
@@ -12,7 +12,7 @@ import { Label } from "@/components/ui/label";
 import { PasswordStrength } from "@/components/ui/password-strength";
 import { useTranslations } from "@/lib/i18n/context";
 import { detectBrowserTimezone } from "@/lib/tz/format";
-import { ApiError, apiFetchRaw, apiPost } from "@/lib/api/api-fetch";
+import { ApiError, apiFetch, apiPost } from "@/lib/api/api-fetch";
 import { clearCachesForSessionEnd, useAuth } from "@/hooks/use-auth";
 
 export default function RegisterPage() {
@@ -90,7 +90,7 @@ export default function RegisterPage() {
     setError(null);
     setSigningOut(true);
     try {
-      await apiFetchRaw("/api/auth/logout", { method: "POST" });
+      await apiFetch("/api/auth/logout", { method: "POST" });
       // Same session-end wipe the shell and the menu do: the QueryClient
       // outlives the navigation on this SPA and the health-data families are
       // not user-scoped, so nothing may survive into the next account.
@@ -105,7 +105,7 @@ export default function RegisterPage() {
       );
       router.refresh();
     } catch {
-      setError(t("common.networkError"));
+      setError(t("auth.signOutFailed"));
     } finally {
       setSigningOut(false);
     }
diff --git a/src/components/auth/__tests__/claim-profile.browser.test.tsx b/src/components/auth/__tests__/claim-profile.browser.test.tsx
index 210081d0d..b4a80a369 100644
--- a/src/components/auth/__tests__/claim-profile.browser.test.tsx
+++ b/src/components/auth/__tests__/claim-profile.browser.test.tsx
@@ -21,11 +21,13 @@ import { recordHistory } from "../../../../tests/browser/url";
 
 const clearOffline = vi.hoisted(() => vi.fn(async () => {}));
 const loadDocument = vi.hoisted(() => vi.fn());
+const showErrorToast = vi.hoisted(() => vi.fn());
 
 vi.mock("@/lib/pwa/query-persister", () => ({
   clearOfflineCachesForSessionEnd: clearOffline,
 }));
 vi.mock("@/lib/navigation/load-document", () => ({ loadDocument }));
+vi.mock("@/lib/errors/error-toast", () => ({ showErrorToast }));
 
 import { ClaimProfile } from "../claim-profile";
 
@@ -57,6 +59,7 @@ beforeEach(() => {
 afterEach(() => {
   clearOffline.mockClear();
   loadDocument.mockClear();
+  showErrorToast.mockClear();
 });
 
 async function fill(selector: string, value: string): Promise<void> {
@@ -112,3 +115,29 @@ describe("ClaimProfile", () => {
     );
   });
 });
+
+describe("ClaimProfile, already signed in", () => {
+  it("keeps the session's page and says so when the server refuses the sign-out", async () => {
+    api.on("POST", "/api/auth/logout", {
+      status: 403,
+      error: "Demo mode: modifications are disabled",
+      meta: { demo: true, errorCode: "demo.readOnly" },
+    });
+    const { queryClient } = await renderWithApp(
+      <ClaimProfile token={TOKEN} />,
+      {
+        user: { username: "signed-in" },
+      },
+    );
+
+    await click("claim-sign-out");
+
+    await expect.poll(() => showErrorToast.mock.calls.length).toBe(1);
+    expect(showErrorToast.mock.calls[0][0]).toBe(
+      "Could not sign out. You are still signed in.",
+    );
+    expect(clearOffline).not.toHaveBeenCalled();
+    expect(queryClient.getQueryData(queryKeys.authMe())).toBeDefined();
+    expect(window.location.search).toBe("");
+  });
+});
diff --git a/src/components/auth/claim-profile.tsx b/src/components/auth/claim-profile.tsx
index d15a6db16..54120d064 100644
--- a/src/components/auth/claim-profile.tsx
+++ b/src/components/auth/claim-profile.tsx
@@ -14,7 +14,8 @@ import { Label } from "@/components/ui/label";
 import { Logo } from "@/components/ui/logo";
 import { PasswordStrength } from "@/components/ui/password-strength";
 import { clearCachesForSessionEnd, useAuth } from "@/hooks/use-auth";
-import { ApiError, apiFetchRaw } from "@/lib/api/api-fetch";
+import { ApiError, apiFetch } from "@/lib/api/api-fetch";
+import { showErrorToast } from "@/lib/errors/error-toast";
 import { useFormatters, useTranslations } from "@/lib/i18n/context";
 import { loadDocument } from "@/lib/navigation/load-document";
 import { clearOfflineCachesForSessionEnd } from "@/lib/pwa/query-persister";
@@ -78,7 +79,7 @@ export function ClaimProfile({
   async function signOut() {
     setSigningOut(true);
     try {
-      await apiFetchRaw("/api/auth/logout", { method: "POST" });
+      await apiFetch("/api/auth/logout", { method: "POST" });
       clearCachesForSessionEnd(queryClient);
       // Back to this page with the link intact: the token only rides the URL.
       router.replace(
@@ -87,6 +88,8 @@ export function ClaimProfile({
           : "/auth/claim",
       );
       router.refresh();
+    } catch (failure) {
+      showErrorToast(t("auth.signOutFailed"), { cause: failure });
     } finally {
       setSigningOut(false);
     }
@@ -98,8 +101,7 @@ export function ClaimProfile({
       { token, username },
       {
         onSuccess: async ({ redirectTo }) => {
-          // Only the path and query of the server's answer, so a response
-          // could never send the browser off this origin.
+          // Path and query only, so an answer can never leave this origin.
           const next = new URL(redirectTo, window.location.origin);
           if (clientTransport) {
             next.searchParams.set("client_proto", clientTransport.protocol);
@@ -120,8 +122,7 @@ export function ClaimProfile({
       { token, username, email, password },
       {
         onSuccess: () => {
-          // As after any sign-in: nothing cached before belongs to this account.
-          queryClient.clear();
+          queryClient.clear(); // as after any sign-in
           router.replace("/onboarding/handover");
         },
       },