The culling system blocking is likely just a symptom of something else. It works by polling the GPU until the work is done. If something else is holding up the GPU, it’s going to show as the flush blocking until whatever is holding it up is done.
I remember having similar issues with SteamVR in UE4 (although without crashing), and could never figure out the cause.
The first thing to check is if your render is taking up that time. If not, profiling with something like Nvidia Nsight Systems might help seeing what is using the GPU.