Fix blank page when a startup request fails

ui/app/manager/src/index.ts

@@ -171,6 +171,10 @@ fetch(configURL)
 
     return { ...appConfig };
   })
+  .catch((e) => {
+    console.error("Failed to load the manager app config, using the defaults", e);
+    return {} as ManagerAppConfig;
+  })
   .then((appConfig: ManagerAppConfig) => {
     // Set locales and load path
     if (!appConfig.manager) {

ui/app/manager/test/startup.test.ts

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2026, OpenRemote Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+import { expect } from "@openremote/test";
+import { test } from "./fixtures/manager.js";
+
+// What a reverse proxy answers while the manager restarts
+const serviceUnavailable = {
+  status: 503,
+  contentType: "text/html",
+  body: "<html><body><h1>503 Service Unavailable</h1></body></html>",
+};
+
+/**
+ * @given The manager app config can't be fetched
+ * @when The manager is opened
+ * @then The login page is shown
+ */
+test("should show the login page when the app config can't be fetched", async ({ page, manager }) => {
+  await page.route("**/api/master/configuration/manager", (route) => route.abort("connectionreset"));
+  await manager.goToRealmStartPage("master");
+  // The app was never added to the page, which stayed blank
+  await expect(page.getByRole("textbox", { name: "Username or email" })).toBeVisible();
+});
+
+/**
+ * @given The manager info endpoint answers with an HTML error page
+ * @when The manager is opened
+ * @then The login page is shown
+ */
+test("should show the login page when the manager info is not JSON", async ({ page, manager }) => {
+  await page.route("**/api/master/info", (route) => route.fulfill(serviceUnavailable));
+  await manager.goToRealmStartPage("master");
+  // Initialization waited forever for the info, which left the page blank
+  await expect(page.getByRole("textbox", { name: "Username or email" })).toBeVisible();
+});

ui/component/core/src/index.ts

@@ -457,7 +457,11 @@ export class Manager implements EventProviderFactory {
       const json = await new Promise<any>((resolve, reject) => {
         const oReq = new XMLHttpRequest();
         oReq.addEventListener("load", () => {
-          resolve(JSON.parse(oReq.responseText));
+          try {
+            resolve(JSON.parse(oReq.responseText));
+          } catch (e) {
+            reject(e);
+          }
         });
         oReq.addEventListener("error", () => {
           reject(new Error("Failed to contact the manager"));