C++ Share Library between dll and exe

I am using The GNU MP (MPIR) Library and i want to Use the provided mpf_class as an argument of an dynamically loaded function from a dll. My problem is, that in general the dll import and running of functions work, but as soon as I try to pass a mpf_class as function argument, i get a Runtime Expression saying : `

Exception thrown at 0x7BB6E491 in Fractal Explorer.exe: 0xC0000005: Access violation executing location 0x7BB6E491.

I am linking the GMP Library with both the executeable and the dll.

This is the code of the exe that fails:

#pragma once

#include <windows.h>
#include <mpirxx.h>
#include <iostream>
#include <utility>
#include <limits.h>


typedef long (__stdcall *f_lfunc)(mpf_class const_r, mpf_class const_c, unsigned long max_iter, unsigned long precision);
typedef int (__stdcall* f_ifunc)();

const wchar_t* GetWC(const char* c);

class DllFractal {
public:
    DllFractal(const char *path) {
        wchar_t* buf = (wchar_t *) GetWC(path);

        hGetProcIDDLL = LoadLibrary(buf);
        if (!hGetProcIDDLL) {
            std::cout << "[DLL LOADER] Could not Load DLl : \"" << buf << "\"!" << std::endl;
            unsigned int err = GetLastError();
            std::cout << ": -------------------- errorcode: " << err << std::endl;
            return;
        }

        get_iterations = (f_lfunc)GetProcAddress(hGetProcIDDLL, "get_iterations");

        if (!get_iterations) {
            std::cout << "[DLL LOADER] Could not locate the function \"get_iterations()\"" << std::endl;
            unsigned int err = GetLastError();
            std::cout << ": -------------------- errorcode: " << err << std::endl;
            return;
        }
    }
    virtual ~DllFractal() {
        FreeLibrary(hGetProcIDDLL);
    }
    long getIterations(mpf_class r, mpf_class c, unsigned long max_iter, unsigned long precision) {

/*
       This is where it fails!
*/      
       return get_iterations(r, c, max_iter, precision); 
    }
private:
    f_lfunc get_iterations;
    HINSTANCE hGetProcIDDLL;
};

const wchar_t* GetWC(const char* c)
{
    size_t cSize = strlen(c) + 1;
    size_t converted;
    wchar_t* wc = new wchar_t[cSize];
    mbstowcs_s(&converted, wc, cSize, c, cSize - 1);
    return wc;
}

And this is the code of the function i'm trying to call in the dll :

unsigned long get_iterations(mpf_class const_r, mpf_class const_c, unsigned long max_iter, unsigned long precision) {
    mpf_class two(2, precision);
    mpf_class escape(4, precision);

    mpf_class r(0, precision);
    mpf_class c(0, precision);

    mpf_class temp_r(0, precision);
    unsigned long i;
    for (i = 0; i < max_iter; i++) {
        if (r * r + c * c > escape) {
            break;
        }

        // f(z) = z^2 + c
        temp_r = r;
        r = temp_r * temp_r - c * c + const_r;
        c = two * temp_r * c + const_c;

    }
    return i;
}

thanks in advance and if you need more information please tell me.