Hello, thanks in advance for your code.
I referred to it, and I also met a rendering exception, not sure whether the same as you met, when I load map for the second time.
I also found LoadingWidget became unusable for the second time as I play in viewport, and that widget is destroyed on loading map.
Then I put LoadingWidget = CreateWidget(this, wLoading); in BeginLoadingScreen, and no crash occurs either in PlayInViewport or in Standalone. It works.
PS. My MoviePlayer only works in Standalone (screen stuck in PlayInViewport until map is loaded). Are yours the same?