How do i remove an error message coming from an external CSS ?

Greetings,

I have no ideas what's causing that, but when I'm logged in and I'm watching pages of my main site (the home site of my WPMU/Buddypress network), I have an orange "Error" text displaying :
https://dl.dropboxusercontent.com/u/5140132/wpmu/css-error-message.png

I don't want my users to see it, so how do I remove it ?

For your information, if it's helpful, I searched that message with Firebug, and here's what I got :
https://dl.dropboxusercontent.com/u/5140132/wpmu/css-error-message-firebug.png

So if I'm not mistaken, the message seems to come from an external CSS file on the "akamahid.net" server... and I have no idea what it is (maybe something of an old web-designer of ours ?).

Anyway, what's the best way to remove that "Error" text so that it doesn't display on my pages ?

If you want to see it in action, it's here (or in any other page) :
http://www.ageneve.net/v15/a-propos/

It doesn't seem to display when you're not loggedin, neither when you're logged in as a simple user... Weird isn'it ?

Any idea ?

Thanks a lot for your help !

DJ