MS Visual C++ Multithreading accessing allocated variables causes access violation

Question

I have an issue with an MFC dialog based application build with MSVC 2013. To make the main dialog accessible also during more elaborate functions, I'm using multi-threading. A click on a button in the dialog calls a "worker function" that is worked out by another thread.

Here's an excerpt of the class:

class CpiezcamDlg : public CDialogEx
{
protected:
    virtual BOOL OnInitDialog();

public:
    CWinThread *m_thread1;

    void StartSweepAndImageThread()
    {
        m_thread1 = AfxBeginThread(SweepAndImageThreadProc, this);
    }

private:
    static UINT SweepAndImageThreadProc(LPVOID pParam)
    {
        CpiezcamDlg *pThis = (CpiezcamDlg *)pParam;
        UINT nRet = pThis->DoSweepAndImage();
        return nRet;
    }

    UINT DoSweepAndImage();

    UINT16 steps;
    CString *imgs_name;
};

Clicking a button calls StartSweepAndImageThread which itself calls SweepAndImageThreadProc and finally DoSweepAndImage. In the function DoSweepAndImage, variables of the class are accessed (read and write). Amongst others, there is imgs_name. The usage is:

UINT CpiezcamDlg::DoSweepAndImage()
{
    // ...
    CString str;
    str.Format(_T("Test"));
    AddStageListText(str);

    imgs_name[i] = str;
    // ...
}

while imgs_name is initialized like

steps = 4;
imgs_name = new CString[steps];

in the OnInitDialog function.

The problem is that when pressing the mention button I receive

0xC0000005: Access violation reading location 0xFDFDFDF9.

exactly on imgs_name[i] = str;. When using a statical array, that is instead of CString *imgs_name; I define CString imgs_name[4];, everything works well. However, I very much would like to have that CString variable a dynamical one. Thanks in advance for your help!

PS: When I evaluated this in a serial way, i.e. when running the DoSweepAndImage function in the main thread, everything goes well. That's why I assume the access violation is due to the multi-threading.

@Wimmel: The loop over i in DoSweepAndImage is

for (UINT16 i = 0; i < steps; i++)