It’s difficult, but it’s almost certainly related to compiler flags given when building. You said you were definitely building for Win64 (that is often the cause of this kind of thing); another thing to check is the version of the C runtime library used. I think should be multithreaded, can’t recall if it should be static or dynamic. Also I think you should build it using release version of C runtime, and defining NDEBUG. By default, UE4 uses release mode of C runtime even in debug builds, so there may be compatibility issues if you use debug version.