My personal theory is that the Google VR plugin used in UE4.21 and later only works with Daydream compatible phones. It works for me in UE4.20 on a GalaxyJ7 and an old Nexus7, the report mentioned earlier, Unreal Engine Issues and Bug Tracker (UE-65983), has been updated saying, “This issue was encountered during the XR compat pass and crash did not occur on 13832_Galaxy_S9_Mali.”, a Daydream compatible phone.