Double pointer "freeze" on uC

Hello folks!

Iam currently working on a project with the XC2785X-104F Controller, 
uVision 4 and Keil C166 Compiler.
Within my code i have to work with matrices and several matrix 
calculation. Therefor iam working with double pointers, structs and 2 
dimensional arrays.

######################## Code snippet ####################

#################################### matrix struct (incl col,row)

typedef struct {
  int  row;
  int  col;
  }  MATHEAD;

typedef struct {
  MATHEAD  head;
  /*
  * only the starting address of the following will be
  * returned to the C programmer, like malloc() concept
  */
  double  *matrix;
  }  MATBODY;

typedef  double **MATRIX;

#define  Mathead(a)  ((MATHEAD *)((MATHEAD *)(a) - 1))
#define MatRow(a)  (Mathead(a)->row)
#define  MatCol(a)  (Mathead(a)->col)



############################################# struct to work with

typedef struct {
  MATRIX A, B, C;
        double var;

}structure;

############################################ func to "create" matrices

MATRIX  mat_creat( row, col, type )
int row, col, type;
{
  MATRIX  A;

  if ((A =_mat_creat( row, col )) != NULL)
    {
    return (mat_fill(A, type));
    }
  else
    return (NULL);

return (A);
}

MATRIX  _mat_creat( row, col )
int row, col;
{
  MATBODY  *mat;
  int   i;


   if ((mat = (MATBODY *)malloc( sizeof(MATHEAD) + sizeof(double *) * 
row)) == NULL)
    return NULL;//(mat_error( MAT_MALLOC ));

  for (i=0; i<row; i++)
  {
  if ((*((double **)(&mat->matrix) + i) = (double 
*)malloc(sizeof(double) * col)) == NULL)
    return NULL;//(mat_error( MAT_MALLOC ));
  }

  mat->head.row = row;
  mat->head.col = col;

  return (&(mat->matrix));   //(& )
}

###################################################### main.c

structure test1;

// value for var
test1.var = 1.234;

// send data via uart to terminal
send_uart(test1.var);                    // works fine

// create a 2x1 matrix with unknown data
test1.A = mat_creat(2,1,UNDEFINED);

// fill matrix with values
test1.A[0][0] = 0x00;                    // <--------- causes uC-freeze 
at this point


// send data via uart to terminal
send_uart(test1.A[0][0]);                // is not delivered
send_uart("TEST\n");                     // is not delivered



##################### END Code snippet ##############################

If i comment that line out (// test1.A[0][0] = 0x00;) everything works 
fine.
If not, there is no error nor a warning from the compiler. Just a freeze 
on the uC.

Does anybody know where the problem is?
Did anyone make similar experiences??

I hope someone can help me!

thank you!