Bitmap region is already locked, multi-threaded environment

Question

I'm working in an API that calls the main function through multiple threads. I'm trying to access through that function a Bitmap in another class and write from it, but even after setting it to use completely different instance of the object I am experiencing an InvalidOperationException: Bitmap region is already locked.

I've tried locking code in the main function and where Bitmap.LockBits(...) is being called. Yes, UnlockBits is being called when I'm done.

    /* Part of Class B */
    public Surface imageSurface //Surface is a field of pixels, more or less.
    {
        get
        {
            if (_CurrImage != null && _imageSurface == null)
            {

                _imageSurface = Surface.CopyFromBitmap(_CurrImage);
                return Surface.CopyFromBitmap(_imageSurface.CreateAliasedBitmap());
            }
            else
            {
                Surface clearPixel = new Surface(1, 1);
                clearPixel[0, 0] = ColorBgra.FromBgra(255, 255, 255, 0);
                return clearPixel;
            }
        }
    }
    /* the "main" function, Class A */
    public override void Render(ClassBPrototype parameters, ...)
    {
        ClassB token = (ClassB)parameters; // Here we go again~!
        ...
        Surface ourSurface = dstArgs.Surface;
        if (token.imageSurface.Size != null)
        {
            ourSurface = token.imageSurface;
        }

        lock(typeof(ClassA))
        {
            for (int lRectangleIndex = ...)
            {
                Rectangle lRectangle = rois[lRectangleIndex];

                for (int y = ...)
                {
                    surfaceY = (ourSurface.Height / 2) - (y - (int)CenterY);

                    for (int x = ...)
                    {
                        surfaceX = (ourSurface.Width / 2) - (x - (int)CenterX);
                        if (surfaceX >= 0 && surfaceX < ourSurface.Width && surfaceY >= 0 && surfaceY < ourSurface.Height)
                        {
                            dstArgs.Surface[x, y] = ourSurface[surfaceX, surfaceY];
                        }
                        else
                        {
                            dstArgs.Surface[x, y] = ColorBgra.FromBgra(255, 255, 255, 0);
                        }

                    }
                }
            }
        }
    }