What do you mean by that?
This error has happened every day in every scenario since I first installed UE4.
Build From Source
Debug From Source
Pre-compiled from Epic.
I am not working on anything else in Visual Studio.
UnrealLightmass shouldn’t bother UnrealBuildTool at all. TBH there are no mention of it in sources (besides, build scripts).
Are you sure you are not getting this error during some building not running?
I meant the VS2013 error from your last post.