diff --git a/public/_locales/en/messages.json b/public/_locales/en/messages.json
index 6fc90b9..aa03e25 100644
--- a/public/_locales/en/messages.json
+++ b/public/_locales/en/messages.json
@@ -134,6 +134,9 @@
   "uploadSessionExpired": {
     "message": "The encrypted upload session expired before it could finish. Please send the message again."
   },
+  "operationFailed": {
+    "message": "The operation could not be completed. Please try again."
+  },
   "decryptingButton": {
     "message": "Decrypting..."
   }
diff --git a/public/_locales/nl/messages.json b/public/_locales/nl/messages.json
index 7aa61ec..b0dd669 100644
--- a/public/_locales/nl/messages.json
+++ b/public/_locales/nl/messages.json
@@ -134,6 +134,9 @@
   "uploadSessionExpired": {
     "message": "De versleutelde uploadsessie is verlopen voordat deze kon worden voltooid. Verstuur het bericht opnieuw."
   },
+  "operationFailed": {
+    "message": "De bewerking kon niet worden voltooid. Probeer het opnieuw."
+  },
   "decryptingButton": {
     "message": "Ontsleutelen..."
   }
diff --git a/src/pages/yivi-popup/init-flow.ts b/src/pages/yivi-popup/init-flow.ts
index 3b48a99..119521b 100644
--- a/src/pages/yivi-popup/init-flow.ts
+++ b/src/pages/yivi-popup/init-flow.ts
@@ -92,12 +92,16 @@ export async function initCryptoPopup(deps: InitCryptoPopupDeps): Promise<void>
     }, deps.autoCloseDelayMs ?? DEFAULT_AUTO_CLOSE_MS);
   } catch (e) {
     console.error("[PostGuard] Crypto popup error:", e);
+    // Only surface messages from error types we know are safe to show.
+    // Raw SDK error text can embed internal server URLs or subsystem
+    // names, so it is logged above (console.error) but never forwarded
+    // to the popup UI or sent to the background — the user sees a
+    // generic fallback instead. Add new cases here only for error types
+    // whose `.message` is verified safe for display.
     const message =
       e instanceof UploadSessionExpiredError
         ? deps.i18n.getMessage("uploadSessionExpired")
-        : e instanceof Error
-        ? e.message
-        : "Operation failed.";
+        : deps.i18n.getMessage("operationFailed");
     await deps.sendError(windowId, message).catch(() => undefined);
     deps.ui.showError(message);
   }
diff --git a/tests/crypto-popup.test.ts b/tests/crypto-popup.test.ts
index 490abfd..19a5581 100644
--- a/tests/crypto-popup.test.ts
+++ b/tests/crypto-popup.test.ts
@@ -1,4 +1,5 @@
 import { describe, it, expect, beforeEach, vi } from "vitest";
+import { UploadSessionExpiredError } from "@e4a/pg-js";
 import { buildRecipients } from "../src/pages/yivi-popup/recipients";
 import { runEncryptInPopup, YIVI_ELEMENT_SELECTOR } from "../src/pages/yivi-popup/encrypt-popup";
 import { runDecryptInPopup } from "../src/pages/yivi-popup/decrypt-popup";
@@ -556,7 +557,9 @@ describe("crypto popup — error handling", () => {
       i18n: { getMessage: (k) => k },
       scheduleAutoClose: () => undefined,
     });
-    expect(sendError).toHaveBeenCalledWith(8, "encrypt boom");
+    // Raw SDK error text ("encrypt boom") must NOT leak — the generic
+    // i18n fallback is sent to the background instead.
+    expect(sendError).toHaveBeenCalledWith(8, "operationFailed");
   });
 
   it("should send error message to background on decrypt failure", async () => {
@@ -577,7 +580,9 @@ describe("crypto popup — error handling", () => {
       i18n: { getMessage: (k) => k },
       scheduleAutoClose: () => undefined,
     });
-    expect(sendError).toHaveBeenCalledWith(9, "decrypt boom");
+    // Raw SDK error text ("decrypt boom") must NOT leak — the generic
+    // i18n fallback is sent to the background instead.
+    expect(sendError).toHaveBeenCalledWith(9, "operationFailed");
   });
 
   it("should display error in popup UI", async () => {
@@ -597,7 +602,58 @@ describe("crypto popup — error handling", () => {
       i18n: { getMessage: (k) => k },
       scheduleAutoClose: () => undefined,
     });
-    expect(ui.error).toBe("UI boom");
+    // Raw SDK error text ("UI boom") must NOT be shown — the popup UI
+    // displays the generic i18n fallback instead.
+    expect(ui.error).toBe("operationFailed");
+  });
+
+  it("should surface the i18n message for the safe UploadSessionExpiredError type", async () => {
+    const ui = makeUi();
+    const data = fakeData("encrypt");
+    const sendError = vi.fn(async () => undefined);
+    await initCryptoPopup({
+      resolveWindowId: async () => 7,
+      requestInitData: async () => data,
+      createPg: () => ({} as any),
+      runEncrypt: async () => {
+        throw new UploadSessionExpiredError("test-uuid", "expired_or_unknown", "");
+      },
+      runDecrypt: async () => undefined,
+      sendError,
+      closeWindow: async () => undefined,
+      ui,
+      i18n: { getMessage: (k) => k },
+      scheduleAutoClose: () => undefined,
+    });
+    expect(sendError).toHaveBeenCalledWith(7, "uploadSessionExpired");
+    expect(ui.error).toBe("uploadSessionExpired");
+  });
+
+  it("should not leak sensitive SDK error text to the UI or background", async () => {
+    const ui = makeUi();
+    const data = fakeData("encrypt");
+    const sendError = vi.fn(async () => undefined);
+    const secret = "connect ECONNREFUSED https://internal-pkg.corp:8443/v2/request";
+    await initCryptoPopup({
+      resolveWindowId: async () => 3,
+      requestInitData: async () => data,
+      createPg: () => ({} as any),
+      runEncrypt: async () => {
+        throw new Error(secret);
+      },
+      runDecrypt: async () => undefined,
+      sendError,
+      closeWindow: async () => undefined,
+      ui,
+      i18n: { getMessage: (k) => k },
+      scheduleAutoClose: () => undefined,
+    });
+    expect(sendError).toHaveBeenCalledWith(3, "operationFailed");
+    expect(ui.error).toBe("operationFailed");
+    // The internal server URL / subsystem detail must never reach the UI
+    // or the background channel.
+    expect(sendError).not.toHaveBeenCalledWith(3, secret);
+    expect(ui.error).not.toContain("internal-pkg.corp");
   });
 
   it("should not auto-close on error", async () => {