Simply put, if a RetainerBox widget is rotated, via SetRenderAngle, it loses resolution and crashes if the angle exceeds +90 or -90 degrees. Also happens if the RetainerBox is wrapped in another widget and that widget is rotated instead.
[2017.03.28-19.37.50:402][520]LogWindows:Error: Assertion failed: MaxX <= (uint32)D3D11_VIEWPORT_BOUNDS_MAX [File:D:\Build\++UE4+Release-4.15+Compile\Sync\Engine\Source\Runtime\Windows\D3D11RHI\Private\D3D11Commands.cpp] [Line: 240]
[2017.03.28-19.37.50:402][520]LogWindows:Error: Unhandled Exception: EXCEPTION_ACCESS_VIOLATION reading address 0xf3af5940
[2017.03.28-19.37.50:402][520]LogWindows:Error: UE4Editor-SlateCore.dll!TArray<FHittestGrid::FCell,FDefaultAllocator>::RemoveAtImpl() [d:\build\++ue4+release-4.15+compile\sync\engine\source\runtime\core\public\containers\array.h:1407]
...