diff --git a/client/src/components/Files/docsync/DocSyncConnectModal.tsx b/client/src/components/Files/docsync/DocSyncConnectModal.tsx
index 754f6e538..828dfb7b8 100644
--- a/client/src/components/Files/docsync/DocSyncConnectModal.tsx
+++ b/client/src/components/Files/docsync/DocSyncConnectModal.tsx
@@ -1,12 +1,13 @@
 import { useId, useState } from 'react'
 import { AlertTriangle, Check, Loader2, Plug, ShieldAlert } from 'lucide-react'
 import { DialogButton, DialogFooter, DialogHeader, DialogShell, DialogTile, FooterSpacer, NEUTRAL_TINT } from '../../shared/DialogShell'
+import { Tooltip } from '../../shared/Tooltip'
 import ToggleSwitch from '../../Settings/ToggleSwitch'
 import { useTranslation } from '../../../i18n/TranslationContext'
 import { useConnectForm } from './useConnectForm'
 import { Badge } from './DocSyncBits'
 import { DOCUMENT_PROVIDER_ICONS } from '../../shared/DocumentProviderIcons'
-import type { DocSyncProvider, useDocSync } from './useDocSync'
+import type { DocSyncProvider, DocSyncVerdict, useDocSync } from './useDocSync'
 
 /**
  * Connecting a store, as a dialog rather than a panel that unfolds in place.
@@ -32,7 +33,7 @@ export default function DocSyncConnectModal({
   const Icon = DOCUMENT_PROVIDER_ICONS[provider.id]
 
   const form = useConnectForm(provider, existing, sync)
-  const [verdict, setVerdict] = useState<{ connected: boolean; account?: string; error?: string } | null>(null)
+  const [verdict, setVerdict] = useState<DocSyncVerdict | null>(null)
 
   const test = async () => setVerdict(await form.probe())
   const labelId = useId()
@@ -134,7 +135,7 @@ function Verdict({
   busy,
   failure,
 }: {
-  verdict: { connected: boolean; account?: string; error?: string } | null
+  verdict: DocSyncVerdict | null
   busy: boolean
   /** A call that failed outside the connection test (saving, most of all). */
   failure: string | null
@@ -172,10 +173,16 @@ function Verdict({
     )
   }
   const blocked = verdict.error === 'ssrf_blocked' || verdict.error === 'tls_untrusted'
+  const message = t(`docsync.error.${verdict.error || 'unknown'}`)
+  // The footer shares its row with the buttons, and a refusal cut to "The
+  // credentials were re..." with no way to read the rest left nothing to go on
+  // (#2666). Two lines, and the whole of it with the store's reason on hover.
   return (
-    <span className="flex min-w-0 items-center gap-2 text-caption text-danger">
-      {blocked ? <ShieldAlert size={14} /> : <AlertTriangle size={14} />}
-      <span className="truncate">{t(`docsync.error.${verdict.error || 'unknown'}`)}</span>
-    </span>
+    <Tooltip label={verdict.detail ? `${message} ${verdict.detail}` : message} placement="top">
+      <span className="flex min-w-0 items-center gap-2 text-caption text-danger">
+        {blocked ? <ShieldAlert size={14} className="flex-none" /> : <AlertTriangle size={14} className="flex-none" />}
+        <span className="line-clamp-2 [overflow-wrap:anywhere]">{message}</span>
+      </span>
+    </Tooltip>
   )
 }
diff --git a/client/src/components/Files/docsync/useConnectForm.ts b/client/src/components/Files/docsync/useConnectForm.ts
index 4e6d2ea6e..4905f2b41 100644
--- a/client/src/components/Files/docsync/useConnectForm.ts
+++ b/client/src/components/Files/docsync/useConnectForm.ts
@@ -1,5 +1,5 @@
 import { useState } from 'react'
-import type { DocSyncConnection, DocSyncProvider, useDocSync } from './useDocSync'
+import type { DocSyncConnection, DocSyncProvider, DocSyncVerdict, useDocSync } from './useDocSync'
 
 /**
  * The credential form's rules, for both shells.
@@ -39,7 +39,7 @@ export interface ConnectForm {
   setInsecureTls: (on: boolean) => void
   /** Every required field is either filled in or already stored. */
   complete: boolean
-  probe: () => Promise<{ connected: boolean; account?: string; error?: string }>
+  probe: () => Promise<DocSyncVerdict>
   save: () => Promise<boolean>
 }
 
diff --git a/client/src/components/Files/docsync/useDocSync.ts b/client/src/components/Files/docsync/useDocSync.ts
index 9cd8564f8..8f1861451 100644
--- a/client/src/components/Files/docsync/useDocSync.ts
+++ b/client/src/components/Files/docsync/useDocSync.ts
@@ -12,6 +12,14 @@ import { useServerPing } from '../../../sync/useServerPing'
  * reason for the same problem.
  */
 
+/** What a connection test answers; `detail` is the store's own reason, for the person fixing it. */
+export interface DocSyncVerdict {
+  connected: boolean
+  account?: string
+  error?: string
+  detail?: string
+}
+
 export interface DocSyncField {
   field_key: string
   label: string
@@ -264,11 +272,9 @@ export function useDocSync(tripId: number | string, enabled: boolean) {
   const testConnection = useCallback(async (providerId: string, baseUrl: string, credentials: Record<string, string>, allowInsecureTls: boolean) => {
     setBusy('test')
     try {
-      return await docsyncApi.testConnection(tripId, { providerId, baseUrl, credentials, allowInsecureTls }) as {
-        connected: boolean; account?: string; error?: string; detail?: string
-      }
+      return await docsyncApi.testConnection(tripId, { providerId, baseUrl, credentials, allowInsecureTls }) as DocSyncVerdict
     } catch (e: unknown) {
-      return { connected: false, error: readError(e) }
+      return { connected: false, error: readError(e) } satisfies DocSyncVerdict
     } finally {
       setBusy(null)
     }
diff --git a/client/src/mobile/screens/trip/tabs/MDocSyncSheet.tsx b/client/src/mobile/screens/trip/tabs/MDocSyncSheet.tsx
index fd239d0aa..43b52e25a 100644
--- a/client/src/mobile/screens/trip/tabs/MDocSyncSheet.tsx
+++ b/client/src/mobile/screens/trip/tabs/MDocSyncSheet.tsx
@@ -12,7 +12,7 @@ import { useTranslation } from '../../../../i18n'
 import { DOCUMENT_PROVIDER_ICONS } from '../../../../components/shared/DocumentProviderIcons'
 import TrekIcon from '../../../../components/shared/TrekIcon'
 import {
-  bindingNotice, needsReauth, storeName, useDocSync, type DocSyncLink, type DocSyncProvider,
+  bindingNotice, needsReauth, storeName, useDocSync, type DocSyncLink, type DocSyncProvider, type DocSyncVerdict,
 } from '../../../../components/Files/docsync/useDocSync'
 import { useConnectForm } from '../../../../components/Files/docsync/useConnectForm'
 import { conflictPolicyKey, nextConflictPolicy } from '../../../../components/Files/docsync/DocSyncBits'
@@ -450,7 +450,7 @@ function ConnectView({
 }) {
   const { t } = useTranslation()
   const form = useConnectForm(provider, sync.connectionFor(provider.id), sync)
-  const [verdict, setVerdict] = useState<{ connected: boolean; account?: string; error?: string } | null>(null)
+  const [verdict, setVerdict] = useState<DocSyncVerdict | null>(null)
 
   return (
     <div className="mt-3 flex flex-col gap-3">
@@ -503,6 +503,10 @@ function ConnectView({
               ? t('docsync.connect.okAs', { account: verdict.account })
               : t('docsync.connected')
             : t(`docsync.error.${verdict.error ?? 'unknown'}`)}
+          {/* The store's own reason, so a refusal can be told apart from a typo (#2666). */}
+          {!verdict.connected && verdict.detail && (
+            <span className="mt-0.5 block text-m-muted [overflow-wrap:anywhere]">{verdict.detail}</span>
+          )}
         </p>
       )}
 
diff --git a/server/src/nest/doc-sync/providers/synology-drive.client.ts b/server/src/nest/doc-sync/providers/synology-drive.client.ts
index dcb39ea54..60008dcac 100644
--- a/server/src/nest/doc-sync/providers/synology-drive.client.ts
+++ b/server/src/nest/doc-sync/providers/synology-drive.client.ts
@@ -780,8 +780,11 @@ export class SynologyDriveClient {
       api: 'SYNO.API.Auth',
       version: '6',
       method: 'login',
-      account: JSON.stringify(creds.username),
-      passwd: JSON.stringify(creds.password),
+      // Plain, unlike the FileStation parameters below. entry.cgi reads a value
+      // as JSON where it can, auth.cgi takes it as it comes, so a quoted account
+      // logged in as `"trek"`, quotes included, and every account was refused (#2666).
+      account: creds.username,
+      passwd: creds.password,
       session: SESSION_NAME,
       format: 'sid',
       client: 'browser',
diff --git a/server/tests/unit/nest/doc-sync/synology-drive.provider.test.ts b/server/tests/unit/nest/doc-sync/synology-drive.provider.test.ts
index fa5d1c8c3..4a3a51d46 100644
--- a/server/tests/unit/nest/doc-sync/synology-drive.provider.test.ts
+++ b/server/tests/unit/nest/doc-sync/synology-drive.provider.test.ts
@@ -286,7 +286,7 @@ describe('SynologyDriveDocumentProvider: capabilities', () => {
 // ── login and session ────────────────────────────────────────────────────────
 
 describe('SynologyDriveDocumentProvider: login', () => {
-  it('SYNO-PROVIDER-010: logs in on auth.cgi with JSON-quoted values and asks for a CSRF token', async () => {
+  it('SYNO-PROVIDER-010: logs in on auth.cgi with plain values and asks for a CSRF token (#2666)', async () => {
     route({ 'SYNO.FileStation.Info:get': ok({ hostname: 'nas' }) });
     const result = await provider.probe(connection());
 
@@ -296,8 +296,9 @@ describe('SynologyDriveDocumentProvider: login', () => {
       api: 'SYNO.API.Auth',
       version: '6',
       method: 'login',
-      account: '"trek"',
-      passwd: '"nas-secret"',
+      // auth.cgi does not read JSON: a quoted account is a user named "trek" with quotes.
+      account: 'trek',
+      passwd: 'nas-secret',
       session: 'FileStation',
       format: 'sid',
       enable_syno_token: 'yes',
@@ -643,7 +644,7 @@ describe('SynologyDriveDocumentProvider: device tokens', () => {
     store.password = 'rotated';
     restart();
     await provider.listScopes(saved(store));
-    expect(logins()[0].params).toMatchObject({ device_id: 'DEVICE-111111', passwd: '"rotated"' });
+    expect(logins()[0].params).toMatchObject({ device_id: 'DEVICE-111111', passwd: 'rotated' });
     expect(logins()[0].params.otp_code).toBeUndefined();
 
     delete store.otp_code;