[Vince]

There is a certain maximum length for include paths, although I don't know it by heart. I believe it was 32 but it could be less, still. If the path is longer then it is truncated, so what the compiler sees might as well just be "../include/callbacks/OnGameMode", which obviously results in all kinds of glitchy behavior. In any case, try using backslashes instead of forward slashes.

Although I must say that separating a gamemode like that is a terrible design decision. Rather separate by functionality than by type; e.g. all house related stuff in one file and all business related stuff in another file.