If you google around a bit you can find quite a few pages stating mod_security as the reason for this problem. The recommended solution is to add the following to your .htaccess:
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
Please note that I don't know whether doing this is a good idea as I don't have the slightest clue what mod_security is actually doing. On the other hand this htaccess addition was recommended for drupal, joomla, wordpress ... so if you want to use a cms it seems that you have to live with it.