Illegal choice imagefield_widget

hi, this is the first time i'm trying to use imagefield, to create a new content-type.

here's the details:

CCK 6.x-2.6
ImageField 6.x-3.2
/tmp and /files are both writable by www user.
public downloads available.

after creating a new content-type, i go to the edit page, and open "Manage Fields".
in the "Add new field" row, i typed in the label name (Image1), field name (_image1), data type (File), and form element (Image).

when i click "Save", it says:
"An illegal choice has been detected. Please contact the site administrator."

when i open /admin/reports/dblog, there's a log like this:
type: form
location/referer: /admin/content/node-type/header-image/fields
message: Illegal choice imagefield_widget in _add_new_field element.
severity: error

here's my list of enabled modules;
Media RSS, acl, Advanced Forum, Advanced help, Aggregator, Author Pane, bitcache, block, Block Title Link, Block Theme, browscap, color, comment, contact, content, Content Permissions, counter, ctools, Custom Pagers, DAV API (dav), DAV file system, Database logging, Dynamic display block, Read More link, file, Antivirus, Archives, Attachments, Audio files, file_browser, file_cck, Converters, Documents, file_embed, file_gallery, file_image, Restrictions, Slideshows, Spreadsheets, file_text, file_video, file_views, Filebrowser, filefield, FileField Meta, File taxonomy server, filter, Formatted title, forum, Forum Access, getid3, Google Analytics, Header Image, help, Advanced help example, image, Image Attach, Image Gallery, ImageMagick Advanced, Image Import, imageapi, ImageAPI GD2, ImageField, imce, Image assist, jQuery Update, Javascript tools, lightbox2, LoginToboggan, Mail Editor, menu, mimemail, Mime Mail CSS, Mobile Tools, node, Option Widgets, Page manager, panels, Mini panels, Panel nodes, path, pathauto, Pathologic, php, Print, profile, rdf, Safari Search, search, Service links, SimpleFeed, SimpleFeed Item, Statistics, system, taxonomy, Taxonomy Menu, text, token, Token actions, update, user, User Import, userplus, views, Views content panes, Views exporter, Views Slideshow, Views slideshow: ddblock, views_ui, wurfl

please help :)
thank you