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
You might need assistence from your host on this. If this does not solve your problem, ask your host to inspect the webserver errorlog to find the cause of this.