Cannot get executeSync working under Unity for Android app

[volion80]

ffmpeg-kit-min:6.0-2.LTS
Unity 6

The apk is built Ok. The command like '-version' using 'execute' method works fine. But when I try to encode video using 'executeAsync' it fails.
Please help!

Logcat:
2025.03.22 23:22:43.316 7573 7677 Error Unity at com.unity3d.player.ReflectionHelper.getMethodID(SourceFile:0)
2025.03.22 23:22:43.316 7573 7677 Error Unity at UnityEngine._AndroidJNIHelper.GetMethodID (System.IntPtr jclass, System.String methodName, System.String signature, System.Boolean isStatic) [0x00000] in <00000000000000000000000000000000>:0
2025.03.22 23:22:43.316 7573 7677 Error Unity at UnityEngine.AndroidJavaObject._CallStatic (System.String methodName, System.Object[] args) [0x00000] in <00000000000000000000000000000000>:0
2025.03.22 23:22:43.316 7573 7677 Error Unity at UnityEngine.Events.UnityEvent.Invoke () [0x00000] in <000

C# code:

void RunFFmpegCommandAsync(string command)
{
using (AndroidJavaClass configClass = new AndroidJavaClass("com.arthenica.ffmpegkit.FFmpegKitConfig")){
AndroidJavaObject paramVal = new AndroidJavaClass("com.arthenica.ffmpegkit.Signal").GetStatic("SIGXCPU");
configClass.CallStatic("ignoreSignal", new object[] { paramVal });

        using (AndroidJavaClass ffmpeg = new AndroidJavaClass("com.arthenica.ffmpegkit.FFmpegKit"))
        {
            FFmpegSessionCompleteCallback callback = new FFmpegSessionCompleteCallback();
            ffmpeg.CallStatic("executeAsync", new object[] { command, callback });
        }
    }
}

class FFmpegSessionCompleteCallback : AndroidJavaProxy
{
    public FFmpegSessionCompleteCallback() : base("com.arthenica.ffmpegkit.FFmpegSessionCompleteCallback") { }

    void apply(AndroidJavaObject session)
    {
        string returnCode = session.Call<AndroidJavaObject>("getReturnCode").Call<string>("toString");
        string failStackTrace = session.Call<string>("getFailStackTrace");

        Debug.Log($"FFmpeg session completed. Return Code: {returnCode}");
        if (!string.IsNullOrEmpty(failStackTrace))
        {
            Debug.LogError($"Error: {failStackTrace}");
        }
    }
}