If two projects are both built using the DebugGame solution configuration, the first project will fail to compile in DebugGame again due to Unresolved External errors.
It seems when DebugGame is compiled for the first project, the following is written to C:\Program Files (x86)\Epic Games\4.11\Engine\Intermediate\Build\Win64\UE4\Development\UELinkerFixups\PCH.UELinkerFixupsName.h.cpp:
#include "F:\Unreal Projects[ProjectName]\Intermediate\Build\Win64\UE4\Development\UELinkerFixupsName.h"
After compiling (DebugGame) the second project, this file is updated. The above include is replaced by the same include but for the second project instead of the first (...[SecondProjectName\Intermediate\...) When DebugGame is compiled for the first project again (for the second time) this file is not updated and leads to Unresolved External errors.
Project1 fails second compile
Projects can be compiled in any configuration back and forth
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