I do have version 3.0.4 (at avalonia app):

_sparkle = new SparkleUpdater(
    "https://storage.example.org/appcast.xml", 
    new Ed25519Checker(NetSparkleUpdater.Enums.SecurityMode.Strict, "SECRET")
)
{
    RelaunchAfterUpdate = true,
    CustomInstallerArguments = "/S"
};
_sparkle.UpdateDetected += async (s, e) =>
{
    if (_updateInProgress) return;
    _updateInProgress = true;

    await Dispatcher.UIThread.InvokeAsync(async () =>
    {
        var item = e.AppCastItems[0];
        var vm = new UpdateViewModel(_sparkle, item);
        var dialog = new UpdateDialog();
        vm.ParentWindow = dialog;
        dialog.DataContext = vm;
        await dialog.ShowDialog(this);
        vm.Dispose();

        if (!vm.IsInstalling)
        {
            _updateInProgress = false;
        }
    });
};
await _sparkle.StartLoop(true, true);

On most of the computers works fine but there is one which almost always fails:

24.04.2026 15:16:56 : [Autoupdate] Starting update process
24.04.2026 15:17:01 : [Autoupdate] Download finished: C:\Users\XXX\AppData\Local\Temp\Update_2.0.46.exe
24.04.2026 15:17:01 : [Autoupdate] Preparing to exit
24.04.2026 15:17:23 : [Autoupdate] Starting update process
24.04.2026 15:17:23 : [Autoupdate] Download started: C:\Users\XXX\AppData\Local\Temp\Update_2.0.46.exe
24.04.2026 15:17:49 : [Autoupdate] Download error: Downloaded file has invalid signature!
UpdateViewModel OnDownloadHadError 0
SparkleUpdater OnDownloadFinished 0
d__36 MoveNext 0
AsyncStateMachineBox`1 ExecutionContextCallback 0
24.04.2026 15:18:21 : [Autoupdate] Cancelling update process

From user perspective it looks like the dialog is shown and update is being confirmed by user. Then it tries to restart the app but later comes another attempt to download the file (no user interaction involved here) and it fails with signature.
Any ideas?