Hi valthebault
I disabled css optimization from template manager and ja slideshow lite is working fine now,
As i said in my previous post everything works fine on local updated installation ,so you have some script or module from 3rd part installed causing this behaviour on your website .
also would be better to update T3 framework to latest version released 2.7.1 using ja_extension manager component.
Take always a backup first to do any update
Best Regards