ASPX Workflow Initiation Form doesn't close page when we click on "Start" and does nothing when we click on "Cancel", is there a fix?

Hi,

I have this list workflow that I start manually to copy some information from an element in my list A in another list B. I start the workflow manually by clicking a button that I have made on the element's ribbon toolbar. This button leads to the Workflow initiation form in ASPX. We don't have the licence that allows us to use InfoPath on our servers, so no XSN here for the form.

The form, once opened from the List A element, looks like this :

Image may be NSFW.
Clik here to view.

This all works ok. But when we want to click on one of the buttons of the initiation form that has opened  (Start and Cancel) (in my case "Démarrer" and "Annuler", our SharePoint version has a french language pack), the following happens.

For 'Start' button : The workflow is started and is executed properly, but no visual feedback is given to the user. And IE 8 gives me an javascript error. The initiation form stays open

Détails de l’erreur de la page Web
Agent utilisateur : Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET4.0C; .NET4.0E; InfoPath.3)
Horodateur : Thu, 27 Sep 2012 15:48:29 UTC
Message : Sys.WebForms.PageRequestManagerParserErrorException: Impossible d'analyser le message reçu du serveur. Cette erreur peut avoir plusieurs causes possibles : réponse modifiée par des appels à Response.Write(), filtres de réponse, HttpModules ou activation de trace du serveur.
Détails : Erreur d'analyse à proximité de '<script type='text/j'.
Ligne : 5
Caractère : 62099

For the 'Cancel' option, nothing happens. Is there a way to correct both of these problems? (I'd like 'Start' to close the form and redirect to the list, or to the list element, and Cancel to close the form.)

Thank you in advance.