Jitter c external debugging

I have tried to determine what is causing the troubles in my Jitter external with writing post messages to limit the area where problems occour. It seems that the problem occours arbitrary. When i call the schaum_analysis_loadmatrix method in the schaum.analysis external from my Patch at the part:

 post("allocating gridMatice  %ld", i);
 x->gridMatrices[i] = jit_object_new(_jit_sym_jit_matrix, &info);
 post("gridMatice %ld allocated", i);

sometimes it triggers the "could not allocate matrix" message in the Max Console. But even i use the same initial situation every time i get different results at different i values:

and a second result with the same setting:

My schaum_analysis_loadmatrix method does look like:

t_jit_err schaum_analysis_loadmatrix(t_schaum_analysis* x, t_symbol* s) {
    t_filehandle fh;
    char name[MAX_FILENAME_CHARS];
    t_jit_matrix_info cell_minfo;
    short pathId;
    long * cell_bp, typePtr = 'JIT!', i, xPos, yPos;
    ulong version;
    long filesize, x1, y1, x2, y2, xCellPos, yCellPos, width, height;
    t_jit_matrix_info info;
    long* cellp;

    initMatrix(x);
    version = JIT_BIN_VERSION_1;
    x->nrOfCells = x->rows * x->cols;

    if (0 == open_dialog(name, &pathId, &typePtr, NULL, 0)) {
        path_opensysfile(name, pathId, &fh, READ_PERM);
        if (jit_bin_read_header(fh, &version, &filesize)) {
            error("jit.matrix: improper file format %s", name);
            sysfile_close(fh);
            goto out;
        }
        if (jit_bin_read_matrix(fh, x->cellMatrix)) {
            error("could not read matrix");
            sysfile_close(fh);
            goto out;
        }
        sysfile_close(fh);

        jit_object_method(x->cellMatrix, _jit_sym_getinfo, &cell_minfo);
        jit_object_method(x->cellMatrix, _jit_sym_getdata, &cell_bp);
        post("allocating GridMatrices");
        x->gridMatrices = malloc(sizeof(void*) * x->nrOfCells);
        post("GridMatrices allocated");
        for (i = 0; i < x->nrOfCells; i++) {
            yCellPos = i / x->cols;
            xCellPos = i % x->cols;

            cellp = cell_bp + yCellPos * (&cell_minfo)->dimstride[1] + xCellPos * sizeof(long) * 4;
            x1 = cellp[0];
            y1 = cellp[1];
            x2 = cellp[2];
            y2 = cellp[3];

            width = x2 - x1;
            height = y2 - y1;

            jit_matrix_info_default(&info);
            info.planecount = 1;
            info.dimcount = 3;
            info.dim[0] = width;
            info.dim[1] = height;
            info.dim[2] = x->frames;
            info.type = _jit_sym_char;
            post("allocating gridMatice  %ld", i);
            x->gridMatrices[i] = jit_object_new(_jit_sym_jit_matrix, &info);
            post("gridMatice %ld allocated", i);
        }
    }
out:
    return JIT_ERR_NONE;
}

Does anyone have an idea what could cause the problem while allocating matrices?

And how would you proceed when the Max application crashes while using an self written c-external? How could i figure out what causes the crash in detail?