Hi Arnout,
could you try adding
FSlateRenderer* SlateRenderer = SlateApp.GetRenderer();
FScopeLock ScopeLock(SlateRenderer->GetResourceCriticalSection());
above the tick call in your if.
That’s the order the streaming movie is accessing the resources as well.
I don’t know if you’d need the same thing in the other Tick call considering the fact the flow of code can be different in your loading screen manager (as evidenced by the OnPausedForStreamingChanged calling the UpdateLoadingScreen)