diff --git a/js/app.js b/js/app.js
index 60c1695..642a48c 100644
--- a/js/app.js
+++ b/js/app.js
@@ -8938,7 +8938,15 @@ async function init() {
         Bridge.setPref(SYNC_PREFS.preBackup, e.target.checked ? '1' : ''));
     if (syncSetPwd) syncSetPwd.addEventListener('click', syncSetEncPwdFlow);
     if (syncTest)   syncTest.addEventListener('click', syncTestConnection);
-    if (syncNow)    syncNow.addEventListener('click', runSyncNow);
+    // Guard the user entry point: ignore clicks while a sync is running (no
+    // concurrent syncs) and call runSyncNow() with NO arg — the click event
+    // must not land in its `_attempt` retry counter. Internal retries call
+    // runSyncNow directly, so they're unaffected.
+    if (syncNow) syncNow.addEventListener('click', async () => {
+        if (syncNow.disabled) return;
+        syncNow.disabled = true;
+        try { await runSyncNow(); } finally { syncNow.disabled = false; }
+    });
 
     $('#settingAutoBackupEnabled').addEventListener('change', onToggleAutoBackup);
     $('#autoBackupPickDirBtn').addEventListener('click', pickAutoBackupFolder);