Select type AHAH form doesn't return any answer in 6.x-2.4

Hi ! After an update from 6.x-2.2 to 6.x-2.4, when I select an ad type in the add advertisement form, I do not get the type-specific form for the selected advertise. I'm using drupal 6.26 and I don't know if it is of any help but here are the informations firebug returned about the POST request :

POST ahah 
HTTP/1.1 200 OK
Date: Sat, 09 Jun 2012 08:52:43 GMT
Server: Apache/2.2.21 (Win64) PHP/5.3.10
X-Powered-By: PHP/5.3.10
Expires: Sun, 19 Nov 1978 05:00:00 GMT
Last-Modified: Sat, 09 Jun 2012 08:52:43 GMT
Cache-Control: store, no-cache, must-revalidate, post-check=0, pre-check=0
Content-Length: 32
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: application/json

Request
Accept	text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding	gzip, deflate
Accept-Language	fr,fr-fr;q=0.8,en-us;q=0.5,en;q=0.3
Connection	keep-alive
Cookie	has_js=1; SESSc97b4963b7d55cb0905aeed4745379de=f7c14tsmcu67dk42u4u3cp9hn7; Drupal_l10n_client=0; SESSba2489ff4d25b88dd117579ad53ecde7=i4jnuhcbjidn9jttq5lgrs3kd6
DNT	1
Host	localhost
Referer	http://localhost/en/node/add/ad
User-Agent	Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20100101 Firefox/13.0
Request Headers From Upload Stream
Content-Length	2838
Content-Type	multipart/form-data; boundary=---------------------------2921238217421

POST :
title	
menu[link_title]	
menu[parent]	primary-links:0
menu[weight]	0
probability	       100
teaser_include	1
body	
format	       1
changed	
autoactivate	0
autoexpire	        0
maxviews  	0
maxclicks	        0
form_build_id 	form-a28d0c12608140dcf853eadbadfc5dc8
form_token	 d599a2bcae056e66efdc1d0f4820f215
form_id	 ad_node_form
unique_field_override 	0
adstatus	pending
log	
files[upload]	
pathauto_perform_alias	1	
name  	leelush
date	
status	1

Response :
{ "status": true, "data": null }