C mcrypt works / decrypt not using rijndael 128 (aes)

Question

I'm trying to use a small function to encode a "secret word" into a binary ciphered "string" and then convert this into base64 send it over the internet and decrypt it on the other side.

This is my code:

Encode Function:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "base64.h"


#include <math.h>

int encrypt(
    void* buffer,
    int buffer_len, /* Because the plaintext could include null bytes*/
    char* IV,
    char* key,
    int key_len
){
  MCRYPT td = mcrypt_module_open("rijndael-128", NULL, "cbc", NULL);
  int blocksize = mcrypt_enc_get_block_size(td);
  if( buffer_len % blocksize != 0 ){return 1;}

  mcrypt_generic_init(td, key, key_len, IV);
  mcrypt_generic(td, buffer, buffer_len);
  mcrypt_generic_deinit (td);
  mcrypt_module_close(td);

  return 0;
}

void display(char* ciphertext, int len){
  int v;
  for (v=0; v<len; v++){
    printf("%d", ciphertext[v]);
  }
  printf("\n");
}

int main()
{
  MCRYPT td, td2;
  char * plaintext = "CA7D22F5D7BBB3291487D3EFD935E";
  char* IV = "AAAAAAAAAAAAABBB";
  char *key = "0123456789abcdef";
  int keysize = 32; /* 256 bits */
  char* buffer;
  int buffer_len = 32;
  int* ret;

  buffer = calloc(1, buffer_len);
  strncpy(buffer, plaintext, buffer_len);

  printf("plain:   %s\n", plaintext);

  encrypt(buffer, buffer_len, IV, key, keysize);
  char * mybase64 = base64(buffer, buffer_len, ret);
  printf("b64   : %s\n", mybase64);
  printf("cipher: "); display(buffer , buffer_len);
  printf("buffer: %s\n", buffer);

  return 0;
}

Decode Function:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "base64.h"

#include <mcrypt.h>

#include <math.h>
#include <stdint.h>
#include <stdlib.h>

int decrypt(
    void* buffer,
    int buffer_len,
    char* IV,
    char* key,
    int keysize
){
  MCRYPT td = mcrypt_module_open("rijndael-128", NULL, "cbc", NULL);
  int blocksize = mcrypt_enc_get_block_size(td);
  if( buffer_len % blocksize != 0 ){return 1;}

  mcrypt_generic_init(td, key, keysize, IV);
  mdecrypt_generic(td, buffer, buffer_len);
  mcrypt_generic_deinit (td);
  mcrypt_module_close(td);

  return 0;
}

void display(char* ciphertext, int len){
  int v;
  for (v=0; v<len; v++){
    printf("%d", ciphertext[v]);
  }
  printf("\n");
}

int main()
{
//  MCRYPT td;
  char* IV = "AAAAAAAAAAAAABBB";
  char *key = "0123456789abcdef";
  int keysize = 32; /* 256 bits */
  int buffer_len = 32;
  int* ret;


  char * mybase64 = "0DMkJ3UozhLfYjNav5Ih6rLd+aJua5KNTPHs/m2flLo=";

  char * buffer = unbase64(mybase64, (int)strlen(mybase64)+1, ret);


  printf("cipher: "); display(buffer , buffer_len);
  printf("buffer: %s\n", buffer);
  decrypt(buffer, buffer_len, IV, key, keysize);

  printf("plain: %s\n", buffer);

  return 0;
}

My used base64 functions: https://github.com/superwills/NibbleAndAHalf/blob/master/NibbleAndAHalf/base64.h

The strange thing is, the encoded binary data "looks" the same on the console, also in hex, but it does not get decoded correctly, its just weird characters.

Anyone got an idea ?

EDIT: This is the result when running a test of the program:.

Encode:

user@0F5NV:~/aes$ ./aes 
plain:   CA7D22F5D7BBB3291487D3EFD935E
b64   : EWQ2NxzOGCB1XGARxdfC7kLMGzAGJZ8/sEQf6dThxfk=
cipher: 17100545528-502432117929617-59-41-62-1866-522748637-9763-806831-23-44-31-59-7
buffer: d67? u\`????B?%???D?????

Decode:

user@0F5NV:~/aes$ ./unaes
b64   : EWQ2NxzOGCB1XGARxdfC7kLMGzAGJZ8/sEQf6dThxfk=
cipher: 17100545528-502432117929617-59-41-62-1866-522748637-9763-806831-23-44-31-59-7
buffer: d67? u\`????B?%???D?????
plain: ?Q?X?P??Qa?t?@???wi ?qa?Z

As you see the cipher and buffer are the same in Encoding and Decoding but not the plaintext... Why ?!