So, I started searching on the Internet how to solve my problem. And finally I found that there are some bugs in WordPress 2.9. They are:

Bug tersebut adalah:

• Major bug was simply dismissed
• A related critical bug was only partially fixed)
• A last critical bug (and its child bug) crept in at the last moment

As a result of this:

• RSS widgets are completely broken on servers that are missing both of iconv and multi-byte string functions;
• The curl transport is broken on some servers with an outdated library. This breaks the WP cron, and with it future posting, pings, etc.

Not all sites are affected, of course. Most aren’t, in fact. But for those few who are, the situation is frustrating.

To solve this problem, you need to follow this steps:

1. Download this patch (from semiologic.com):

WordPress 2.9 Patch

2. Extract all files (class-simplepie.php, default-widgets.php, http.php) to wp-includes directory. Replace the old files with the patch files.

3. Done…

Good luck… :)

Related posts:

1. WordPress Error After Automatic Upgrading
2. Wordpress Error: open_basedir restriction in effect
3. WordPress 2.8 Review