well i have found the answer of my problem
if you install discount coupon codes + ultimate seo, there is a problem from ultimate seo if you check the boxe "Output W3C valid URLs (parameter string)?" to true
in this case, if there's an error, checkout_confirmation.php will redirect the user to checkout_process.php?error=...&osCid=...
The problem is &, it is converted in & so it isn't valid anymore.
so the solution is the uncheck the "Output W3C valid URLs (parameter string)?" option
bobby l'ours qui pue du slip