diff --git a/src/app/imex/sync/encryption-enable.service.ts b/src/app/imex/sync/encryption-enable.service.ts
index 7a624c544f..6d555be3be 100644
--- a/src/app/imex/sync/encryption-enable.service.ts
+++ b/src/app/imex/sync/encryption-enable.service.ts
@@ -4,6 +4,8 @@ import { SyncLog } from '../../core/log';
 import { SnapshotUploadService } from './snapshot-upload.service';
 import { OperationEncryptionService } from '../../op-log/sync/operation-encryption.service';
 import { WrappedProviderService } from '../../op-log/sync-providers/wrapped-provider.service';
+import { isCryptoSubtleAvailable } from '../../op-log/encryption/encryption';
+import { WebCryptoNotAvailableError } from '../../op-log/core/errors/sync-errors';
 
 const LOG_PREFIX = 'EncryptionEnableService';
 
@@ -38,6 +40,16 @@ export class EncryptionEnableService {
       throw new Error('Encryption key is required');
     }
 
+    // CRITICAL: Check crypto availability BEFORE deleting server data
+    // to prevent data loss if encryption will fail
+    if (!isCryptoSubtleAvailable()) {
+      throw new WebCryptoNotAvailableError(
+        'Cannot enable encryption: WebCrypto API is not available. ' +
+          'Encryption requires a secure context (HTTPS). ' +
+          'On Android, encryption is not supported.',
+      );
+    }
+
     // Gather all data needed for upload (validates provider)
     const { syncProvider, existingCfg, state, vectorClock, clientId } =
       await this._snapshotUploadService.gatherSnapshotData(LOG_PREFIX);
diff --git a/src/app/imex/sync/import-encryption-handler.service.ts b/src/app/imex/sync/import-encryption-handler.service.ts
index 4a8d1fb7ac..223e836eb2 100644
--- a/src/app/imex/sync/import-encryption-handler.service.ts
+++ b/src/app/imex/sync/import-encryption-handler.service.ts
@@ -6,6 +6,7 @@ import { SyncLog } from '../../core/log';
 import { AppDataComplete } from '../../op-log/model/model-config';
 import { OperationEncryptionService } from '../../op-log/sync/operation-encryption.service';
 import { SnapshotUploadService } from './snapshot-upload.service';
+import { isCryptoSubtleAvailable } from '../../op-log/encryption/encryption';
 
 export interface EncryptionStateChangeResult {
   encryptionStateChanged: boolean;
@@ -132,6 +133,20 @@ export class ImportEncryptionHandlerService {
 
     const { syncProvider, existingCfg, state, vectorClock, clientId } = snapshotData;
 
+    // CRITICAL: Check crypto availability BEFORE deleting server data
+    // to prevent data loss if encryption will fail (only needed when enabling encryption)
+    if (isEncryptionEnabled && !isCryptoSubtleAvailable()) {
+      return {
+        encryptionStateChanged: false,
+        serverDataDeleted: false,
+        snapshotUploaded: false,
+        error:
+          'Cannot enable encryption: WebCrypto API is not available. ' +
+          'Encryption requires a secure context (HTTPS). ' +
+          'On Android, encryption is not supported.',
+      };
+    }
+
     try {
       // 1. Delete all server data (encrypted ops can't mix with unencrypted)
       SyncLog.normal(`${LOG_PREFIX}: Deleting server data...`);
diff --git a/src/app/imex/sync/sync-wrapper.service.ts b/src/app/imex/sync/sync-wrapper.service.ts
index a40c55757c..a1ccff2360 100644
--- a/src/app/imex/sync/sync-wrapper.service.ts
+++ b/src/app/imex/sync/sync-wrapper.service.ts
@@ -15,6 +15,7 @@ import { toObservable } from '@angular/core/rxjs-interop';
 import {
   SyncAlreadyInProgressError,
   LocalDataConflictError,
+  WebCryptoNotAvailableError,
 } from '../../op-log/core/errors/sync-errors';
 import { SyncConfig } from '../../features/config/global-config.model';
 import { TranslateService } from '@ngx-translate/core';
@@ -407,6 +408,16 @@ export class SyncWrapperService {
         // File-based sync: Local data exists and remote snapshot would overwrite it
         // Show conflict dialog to let user choose between local and remote data
         return this._handleLocalDataConflict(error);
+      } else if (error instanceof WebCryptoNotAvailableError) {
+        // WebCrypto (crypto.subtle) is unavailable in insecure contexts
+        // (e.g., Android Capacitor serves from http://localhost)
+        this._providerManager.setSyncStatus('ERROR');
+        this._snackService.open({
+          msg: T.F.SYNC.S.WEB_CRYPTO_NOT_AVAILABLE,
+          type: 'ERROR',
+          config: { duration: 15000 },
+        });
+        return 'HANDLED_ERROR';
       } else if (this._isTimeoutError(error)) {
         this._snackService.open({
           msg: T.F.SYNC.S.TIMEOUT_ERROR,
diff --git a/src/app/op-log/encryption/encryption.ts b/src/app/op-log/encryption/encryption.ts
index 3845dbec92..06dd90f6a7 100644
--- a/src/app/op-log/encryption/encryption.ts
+++ b/src/app/op-log/encryption/encryption.ts
@@ -1,10 +1,50 @@
 import { argon2id } from 'hash-wasm';
+import { WebCryptoNotAvailableError } from '../core/errors/sync-errors';
 
 const ALGORITHM = 'AES-GCM' as const;
 const SALT_LENGTH = 16;
 const IV_LENGTH = 12;
 const KEY_LENGTH = 32;
 
+// ============================================================================
+// WEBCRYPTO AVAILABILITY CHECK
+// ============================================================================
+// WebCrypto (crypto.subtle) is unavailable in insecure contexts:
+// - Android Capacitor: serves from http://localhost (not https)
+// - iOS Capacitor: capacitor:// scheme may not be recognized as secure
+//
+// When WebCrypto is unavailable, encryption operations will fail.
+// Call isCryptoSubtleAvailable() before attempting encryption to show
+// a clear error message to the user.
+// ============================================================================
+
+/**
+ * Checks if WebCrypto API (crypto.subtle) is available in the current context.
+ * Returns false in insecure contexts (http://, some custom schemes like Android Capacitor).
+ */
+export const isCryptoSubtleAvailable = (): boolean => {
+  return (
+    typeof window !== 'undefined' &&
+    typeof window.crypto !== 'undefined' &&
+    typeof window.crypto.subtle !== 'undefined'
+  );
+};
+
+/**
+ * Throws WebCryptoNotAvailableError if crypto.subtle is not available.
+ * Call this at the start of encryption/decryption functions to fail fast
+ * with a clear error message.
+ */
+const assertCryptoSubtleAvailable = (): void => {
+  if (!isCryptoSubtleAvailable()) {
+    throw new WebCryptoNotAvailableError(
+      'WebCrypto API (crypto.subtle) is not available. ' +
+        'Encryption requires a secure context (HTTPS). ' +
+        'On Android, encryption is not supported.',
+    );
+  }
+};
+
 /**
  * Holds a derived CryptoKey along with its salt for reuse across operations.
  * Used to avoid expensive Argon2id key derivation for each operation.
@@ -209,6 +249,7 @@ const decryptArgon = async (data: string, password: string): Promise<string> =>
 };
 
 export const encrypt = async (data: string, password: string): Promise<string> => {
+  assertCryptoSubtleAvailable();
   const enc = new TextEncoder();
   const dataBuffer = enc.encode(data);
   const salt = window.crypto.getRandomValues(new Uint8Array(SALT_LENGTH));
@@ -229,6 +270,7 @@ export const encrypt = async (data: string, password: string): Promise<string> =
 };
 
 export const decrypt = async (data: string, password: string): Promise<string> => {
+  assertCryptoSubtleAvailable();
   try {
     return await decryptArgon(data, password);
   } catch (e) {
@@ -296,6 +338,7 @@ export const deriveKeyFromPassword = async (
   password: string,
   salt?: Uint8Array,
 ): Promise<DerivedKeyInfo> => {
+  assertCryptoSubtleAvailable();
   const actualSalt = salt ?? window.crypto.getRandomValues(new Uint8Array(SALT_LENGTH));
   const key = await _deriveKeyArgon(password, actualSalt);
   return { key, salt: actualSalt };
diff --git a/src/app/t.const.ts b/src/app/t.const.ts
index 8b63b6e3df..0bbd996aad 100644
--- a/src/app/t.const.ts
+++ b/src/app/t.const.ts
@@ -1169,6 +1169,7 @@ const T = {
         ENCRYPTION_PASSWORD_REQUIRED: 'F.SYNC.S.ENCRYPTION_PASSWORD_REQUIRED',
         ENCRYPTION_DISABLED_ON_OTHER_DEVICE:
           'F.SYNC.S.ENCRYPTION_DISABLED_ON_OTHER_DEVICE',
+        WEB_CRYPTO_NOT_AVAILABLE: 'F.SYNC.S.WEB_CRYPTO_NOT_AVAILABLE',
         PERSIST_FAILED: 'F.SYNC.S.PERSIST_FAILED',
         DEFERRED_ACTION_FAILED: 'F.SYNC.S.DEFERRED_ACTION_FAILED',
         REMOTE_DATA_TOO_OLD: 'F.SYNC.S.REMOTE_DATA_TOO_OLD',
diff --git a/src/assets/i18n/en.json b/src/assets/i18n/en.json
index af49945e2e..d09b667083 100644
--- a/src/assets/i18n/en.json
+++ b/src/assets/i18n/en.json
@@ -1127,6 +1127,7 @@
         "DECRYPTION_FAILED": "Failed to decrypt synced data. Please check your encryption password.",
         "ENCRYPTION_PASSWORD_REQUIRED": "Encrypted data received but no encryption password is configured. Please set your encryption password in sync settings.",
         "ENCRYPTION_DISABLED_ON_OTHER_DEVICE": "Encryption was disabled on another device. Your sync settings have been updated.",
+        "WEB_CRYPTO_NOT_AVAILABLE": "Encryption is not available on this device. On Android, encryption requires a secure context (HTTPS) which is not available. Please disable encryption in sync settings or sync from a desktop browser.",
         "PERSIST_FAILED": "Failed to save changes. State may be inconsistent - please reload.",
         "DEFERRED_ACTION_FAILED": "Some changes made during sync could not be saved. Please reload to recover.",
         "REMOTE_DATA_TOO_OLD": "Remote data is too old to be processed. Please update your remote app.",