"URI you submitted has disallowed characters" -- even on example.com/index.php

[eluser]sneakyimp[/eluser]
I'm very new to codeigniter (and enjoying it so far). I am trying to get another developer up to speed on a legacy project. I downloaded the source code, database, etc. to my development machine (Ubuntu 11) and was working happily there with a local installation of apache and an entry in /etc/hosts to refer a temporary url to my local machine.

I zipped up my source code, database, etc. and sent it to the other developer working on a mac with same apache version, same php version, etc. He's using the same db credentials, the same file system path, the same apache configuration, everything and yet codeigniter refuses even the most basic url:

http://example.com/index.php

I have searched around and found this thread which doesn't seem to apply for this simplest of urls. The errors he reports are:

The URI you submitted has disallowed characters.
The requested URL /".$location." was not found on this server.

He says his browser is redirected to http://example.com/".$location."


Does anyone know what the problem could be? I'm wondering if it might be a character encoding issue or something?