This may be due to mod_security.
While mod_security can be a very powerful tool, misconfigured or overly strict rule sets can interfere with vBulletins Operation. Below for Apache users you can use an htaccess file and add a specific rule to disable mod_security.
Make or edit your forum .htaccess file and add the following code
Code:
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
Note, place this file in your main forum directory.
See this also thread:
http://www.vbulletin.com/forum/showthread.php?t=167121