I found out that in my header file MainLib.hpp I had wrong path to my MainDLL.hpp header. after changing the path the program runs ok

#pragma once
#include "../Libs/MainDLL.hpp"
//#include "MainDLL.hpp" - fails to load MainDLL.dll library

namespace mf {
    extern mf::MainFuncPtr MainFunc;

    void InitMF(LPCWSTR Path);

    void UninitMF(void);
}