Hi and thanks for your fast answer Ernest.
Velocity is a great library for animating : it's way faster than jquery. And in this case, i am using it to animate big chunks of the site.
I have not loaded the scripts inside wordpress with a clean declaration (i beleive i should enqueue them somewhere ?) and am writing them directly in the header with a file_get_content (this, to be sure scripts are loaded in a particular order). I don't have a big experience with plugins creation and wanted to stay 'independant' from wordpress by doing so. Maybe it's the reason i have this issue now ?
I cannot give you FTP access sadly, but if we have no choice i will try to create a limited FTP access later. It's just time consuming with our process.
Here is the site we are working on : http://anewmars.com/00_dev/ellipse-avocats.com/
I left the plugin activated, so you can see the error popping.
Is there anything i can do to try to debug this ?