Sadly, I’m seeing the same issue as JoshHD22, OpenXRHMD.cpp Line: 3356.
I also have planar reflections in the scene that are toggled on and off as I enter/leave a trigger volume. As I enter the trigger volume, the game crashes with the assertion failed error. I’ve just upgraded the project from UE4.27.2 but I removed all the VR stuff from that project and created it anew for 5.1.1 Without planar reflection, it seems to work fine.
Planar reflections are incredibly important to the level, so I hope this can be addressed, or I’ll have to go back to 4.27.2 for now.