Why does my Project's code not compile after updating to 4.1?

Hi, I’ve updated to 4.1 from GitHub code and my project with custom code gives me now lots of errors that make evident it has lost connection to source files like GameViewportClient.cpp (functions of which are called in my game mode subclass, for example), so how would I repair this? In 4.0.2 it compiled successfully, but now I can’t open the project in 4.1 because it says the module should be compiled.

OK, so apparently it was just a problem with a custom file subclassing from GameViewportClient, after removing it (not useful any more) it compiles just fine.