Can't generate C++ projects since 4.12.5

This error will occur an ANY project of any kind of name. I just happened to print the error from VSTUnreal, but any .uproject will generate the same issue.

For example, if I create a brand new project called “Foo” using epic launcher, I get immediately when it compiles initially:

UnrealBuildTool Exception: ERROR: Not expecting project Foo.vcxproj to already have a target rules of with configuration name Game (UE4GameTarget) while trying to add: FooTarget

That is what is alarming about the error, it is true for any C++ project.

Would it be possible to just describe, really briefly, what the build tool is trying to do here? What is a UE4GameTarget? Is it a kind of base from which my target must be derived? I feel like if I understood this a little better, I could get to the bottom of the issue right away.