diff --git a/client/android/app/src/main/java/com/liketrek/trek/TrekServerStore.java b/client/android/app/src/main/java/com/liketrek/trek/TrekServerStore.java
index f8bea85e9..82e21b319 100644
--- a/client/android/app/src/main/java/com/liketrek/trek/TrekServerStore.java
+++ b/client/android/app/src/main/java/com/liketrek/trek/TrekServerStore.java
@@ -156,6 +156,24 @@ final class TrekServerStore {
     }
 
     /** Start the next launch on the bundled address screen, even with a server remembered. */
+    private static final String KEY_FAILURE = "last_failure";
+
+    /**
+     * Why the server's page did not load, in the WebView's own terms
+     * ("net::ERR_NAME_NOT_RESOLVED (-2)", "HTTP 503"): the address screen shows
+     * it small under its message, so a report carries the cause.
+     */
+    static void setFailure(Context context, String failure) {
+        prefs(context).edit().putString(KEY_FAILURE, failure).commit();
+    }
+
+    /** The failure behind this address screen, once. */
+    static String takeFailure(Context context) {
+        String failure = prefs(context).getString(KEY_FAILURE, null);
+        if (failure != null) prefs(context).edit().remove(KEY_FAILURE).commit();
+        return failure;
+    }
+
     static void showShellOnce(Context context) {
         prefs(context).edit().putBoolean(KEY_SHELL_ONCE, true).commit();
     }
diff --git a/client/android/app/src/main/java/com/liketrek/trek/TrekShellPlugin.java b/client/android/app/src/main/java/com/liketrek/trek/TrekShellPlugin.java
index be9262cc1..b4eac2759 100644
--- a/client/android/app/src/main/java/com/liketrek/trek/TrekShellPlugin.java
+++ b/client/android/app/src/main/java/com/liketrek/trek/TrekShellPlugin.java
@@ -315,6 +315,8 @@ public class TrekShellPlugin extends Plugin {
         JSObject result = new JSObject();
         result.put("url", url == null ? JSONObject.NULL : url);
         result.put("previousUrl", previous == null ? JSONObject.NULL : previous);
+        String failure = TrekServerStore.takeFailure(getContext());
+        result.put("failure", failure == null ? JSONObject.NULL : failure);
         call.resolve(result);
     }
 
diff --git a/client/android/app/src/main/java/com/liketrek/trek/TrekWebViewClient.java b/client/android/app/src/main/java/com/liketrek/trek/TrekWebViewClient.java
index bad0dc398..a015ec889 100644
--- a/client/android/app/src/main/java/com/liketrek/trek/TrekWebViewClient.java
+++ b/client/android/app/src/main/java/com/liketrek/trek/TrekWebViewClient.java
@@ -143,7 +143,10 @@ class TrekWebViewClient extends BridgeWebViewClient {
         String server = TrekServerStore.getUrl(view.getContext());
         Uri serverUri = server == null ? null : Uri.parse(server);
         if (serverUri != null && TrekClientIdentity.matches(String.valueOf(serverUri.getHost()), serverUri.getPort() > 0 ? serverUri.getPort() : 443, request.getHost(), request.getPort())) {
-            com.liketrek.trek.clientcert.ClientCertWatch.serverAsked(view.getContext(), server, onUnreachable);
+            com.liketrek.trek.clientcert.ClientCertWatch.serverAsked(view.getContext(), server, () -> {
+                TrekServerStore.setFailure(view.getContext(), "client certificate requested");
+                onUnreachable.run();
+            });
         }
     }
 
@@ -168,6 +171,7 @@ class TrekWebViewClient extends BridgeWebViewClient {
                 }, 1500);
                 return;
             }
+            TrekServerStore.setFailure(view.getContext(), error.getDescription() + " (" + error.getErrorCode() + ")");
             onUnreachable.run();
             return;
         }
@@ -189,6 +193,7 @@ class TrekWebViewClient extends BridgeWebViewClient {
         if (!request.isForMainFrame()) {
             super.onReceivedHttpError(view, request, errorResponse);
         } else if (status >= 500 || (status >= 400 && wantedCertificate(request.getUrl()))) {
+            TrekServerStore.setFailure(view.getContext(), "HTTP " + status);
             onUnreachable.run();
         } else if (status >= 400) {
             errorPageUrl = request.getUrl().toString();
diff --git a/client/ios/App/App/TrekBridgeViewController.swift b/client/ios/App/App/TrekBridgeViewController.swift
index 39d0bdf0f..64b1c88fc 100644
--- a/client/ios/App/App/TrekBridgeViewController.swift
+++ b/client/ios/App/App/TrekBridgeViewController.swift
@@ -399,6 +399,22 @@ enum TrekKeyCommands {
 enum TrekServerStore {
     private static let key = "TrekServerURL"
 
+    private static let failureKey = "TrekLastFailure"
+
+    /// Why the server's page did not load ("NSURLErrorDomain -1003"): the
+    /// address screen shows it small under its message, so a report carries
+    /// the cause. Taken once.
+    static func noteFailure(_ error: Error) {
+        let ns = error as NSError
+        UserDefaults.standard.set("\(ns.domain) \(ns.code)", forKey: failureKey)
+    }
+
+    static func takeFailure() -> String? {
+        let failure = UserDefaults.standard.string(forKey: failureKey)
+        UserDefaults.standard.removeObject(forKey: failureKey)
+        return failure
+    }
+
     private static let previousKey = "TrekPreviousServerURL"
 
     private static let checkedKey = "TrekServerCheck"
@@ -813,7 +829,11 @@ final class TrekNavigationProxy: NSObject, WKNavigationDelegate {
     func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
         inner.webView?(webView, didFinish: navigation)
         onPageShown?()
-        if TrekSignInRedirect.onServer(server: TrekServerStore.url, url: webView.url) { serverShown = true }
+        if TrekSignInRedirect.onServer(server: TrekServerStore.url, url: webView.url) {
+            serverShown = true
+            // The server's page is up: an earlier failure is no cause any more.
+            _ = TrekServerStore.takeFailure()
+        }
         if let welcome = TrekServerStore.takeWelcomeURL() {
             webView.load(URLRequest(url: welcome))
         }
@@ -829,6 +849,7 @@ final class TrekNavigationProxy: NSObject, WKNavigationDelegate {
             TrekClientCertWatch.serverAsked(host: space.host, port: space.port) { [weak self, weak webView] in
                 guard let self, let webView else { return }
                 // Capacitor's own answer to a failed load: the address screen (errorPath).
+                TrekServerStore.noteFailure(URLError(.clientCertificateRejected))
                 self.inner.webView?(webView, didFailProvisionalNavigation: nil, withError: URLError(.clientCertificateRejected))
             }
             return
@@ -853,6 +874,7 @@ final class TrekNavigationProxy: NSObject, WKNavigationDelegate {
            http.statusCode >= 400, http.statusCode < 500, let asked = TrekClientIdentity.lastRequest, !asked.served,
            let url = http.url, url.host?.lowercased() == asked.host, (url.port ?? 443) == asked.port {
             decisionHandler(.cancel)
+            TrekServerStore.noteFailure(URLError(.clientCertificateRequired))
             inner.webView?(webView, didFailProvisionalNavigation: nil, withError: URLError(.clientCertificateRequired))
             return
         }
@@ -870,11 +892,13 @@ final class TrekNavigationProxy: NSObject, WKNavigationDelegate {
 
     func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error) {
         if Self.isReplaced(error) { return }
+        TrekServerStore.noteFailure(error)
         inner.webView?(webView, didFail: navigation, withError: error)
     }
 
     func webView(_ webView: WKWebView, didFailProvisionalNavigation navigation: WKNavigation!, withError error: Error) {
         if Self.isReplaced(error) { return }
+        TrekServerStore.noteFailure(error)
         inner.webView?(webView, didFailProvisionalNavigation: navigation, withError: error)
     }
 }
diff --git a/client/ios/App/App/TrekShellPlugin.swift b/client/ios/App/App/TrekShellPlugin.swift
index c00b02ce8..94b641ae2 100644
--- a/client/ios/App/App/TrekShellPlugin.swift
+++ b/client/ios/App/App/TrekShellPlugin.swift
@@ -352,6 +352,7 @@ public class TrekShellPlugin: CAPPlugin, CAPBridgedPlugin, ASWebAuthenticationPr
         call.resolve([
             "url": TrekServerStore.url ?? NSNull(),
             "previousUrl": TrekServerStore.previousURL ?? NSNull(),
+            "failure": TrekServerStore.takeFailure() ?? NSNull(),
         ])
     }
 
diff --git a/client/native-shell/main.ts b/client/native-shell/main.ts
index 503506fa8..341295928 100644
--- a/client/native-shell/main.ts
+++ b/client/native-shell/main.ts
@@ -1546,7 +1546,7 @@ async function connected(url: string, signInHosts: string[] = [], welcome?: stri
   });
 }
 
-function showUnreachable(server: string, cert?: ClientCertDemand): void {
+function showUnreachable(server: string, cert?: ClientCertDemand, failure?: string): void {
   const certPanel = el('div', { className: 'gate', role: 'alert', hidden: true });
   if (cert) fillCertPanel(certPanel, cert, () => void TrekShell.reload());
   const retry = el('button', { type: 'button', className: cert ? 'linkbtn' : 'btn' }, t('native.offline.retry'));
@@ -1576,7 +1576,14 @@ function showUnreachable(server: string, cert?: ClientCertDemand): void {
               : t('native.offline.hint', { server: new URL(server).host })
           ),
         ],
-        [certPanel, retry, change, footLinks()]
+        [
+          certPanel,
+          retry,
+          change,
+          // The cause in the web view's own terms, small: what a report needs, not a second message.
+          ...(failure ? [el('p', { className: 'diag', dir: 'ltr' }, failure.slice(0, 120))] : []),
+          footLinks(),
+        ]
       )
     )
   );
@@ -1664,8 +1671,9 @@ async function start(): Promise<void> {
   letKeyboardClose();
   let url: string | null;
   let previousUrl: string | null | undefined;
+  let failure: string | null | undefined;
   try {
-    ({ url, previousUrl } = await TrekShell.getServer());
+    ({ url, previousUrl, failure } = await TrekShell.getServer());
   } catch {
     // No native shell under us (a web preview, or a simulator without the
     // plugin): there is no remembered server, so start at the welcome screen
@@ -1689,7 +1697,7 @@ async function start(): Promise<void> {
       // On iOS the stored copy of its start page brings the offline-capable UI up anyway.
       const { opened } = await TrekShell.openOfflineCopy();
       if (opened) return;
-      showUnreachable(url);
+      showUnreachable(url, undefined, failure ?? undefined);
     }
   }
   await SplashScreen.hide();
diff --git a/client/native-shell/shell.css b/client/native-shell/shell.css
index 3520a47c1..a71532639 100644
--- a/client/native-shell/shell.css
+++ b/client/native-shell/shell.css
@@ -1556,3 +1556,14 @@ body.quiet::after {
 .plan-feats .plan-photos svg {
   color: var(--faint);
 }
+
+/* The cause of a failed load, for a report: small, quiet, and selectable. */
+.diag {
+  margin: 0;
+  text-align: center;
+  color: var(--faint);
+  font-family: ui-monospace, 'SF Mono', Menlo, Consolas, monospace;
+  font-size: 0.72rem;
+  user-select: text;
+  -webkit-user-select: text;
+}
diff --git a/client/src/native/trekShell.ts b/client/src/native/trekShell.ts
index c5b82d866..23319ddc2 100644
--- a/client/src/native/trekShell.ts
+++ b/client/src/native/trekShell.ts
@@ -18,7 +18,12 @@ export interface TrekShellPlugin {
    * newer one (SERVER_CHECK in TrekServerStore). Set only while no server is,
    * so the address screen can check it again, pre-filled.
    */
-  getServer(): Promise<{ url: string | null; previousUrl?: string | null }>;
+  /**
+   * The server picked, the one before an update asked to check it again, and
+   * why its page did not load last time (taken once; builds before 5.1.4 leave
+   * it out).
+   */
+  getServer(): Promise<{ url: string | null; previousUrl?: string | null; failure?: string | null }>;
   /**
    * Remembers the server and loads it. The current page is gone afterwards.
    * `signInHosts` are the hosts of a proxy's sign-in page in front of it