Steps do reproduce:

1) Download attached project 'vr_mainframe_proj.zip' and open in Unity.
2) Build and run project on a device.
3) Press on this button(see attached image 'button1.png').
4) Now press on this button(see attached image 'button2.png').
Note: Wait until it finish to load and press it again.
5) Place the phone horizontally and it will enter the VR mode.
6) Wait for 5-10 minutes, app will crash and return to main menu.

Reproduced with:

5.4.0f3, 5.4.1f1, 5.5.0b2