The Dreamhost rules for mod_security don't like the url. Unfortunately, DH does not publish (ostensibly for security reasons - security by obscurity?) their exact rules, so I can't say precisely what about the url "breaks" under mod_security .
That said, the easiest fix is to just run your domain without mod_security. You can do this from the control panel-->manage domains--> screen by selecting the "edit" link next to the domain name.
On the screen that results, de-select the "radio button" for "Extra Web Security" (which is what DH calls mod_security), save the form, and wait for the update to take effect (usally 10-20 minutes, though it could be longer or shorter).