I had been dealing with this issue after a merge from 5.0 to 5.1. and it had been driving me up the walls
I am working together with p4v and UGS and sync the game project very regularly but what is left out are the files in the engine folder.
A hint in my crashlog directed me to the EditorLandscapeResources that I had to force sync
Engine/EditorLandscapeResources/CircleBrushMaterial.CircleBrushMaterial
For me this fixed my crashes but I am unsure how to fix this problem with homegrown projects