It sounds like you are trying to use Atelier with client-side source control hooks against a shared development instance - is this correct?  YES 

Also we would always have this issue because we use CCR for code deployment. Even is we move all the developers to use client side hook whenever a CCR is done the code would be modified at server side and then the classes would go out of sync. 

You are right something is wrong in the setup on the specific HS instance, I am guessing something to do with IIS as it is routing the traffic. As I got your WebTerminal application and even that didn't work on that instance but worked on other instance. 

I will try to create a simple REST application with just GET and try to call it and see  what is wrong. 

On a side note what is Caché Visual Editor and where to get it from.