Sharepoint 2013 app: error on postback

I've deployed a provider hosted app (hosted on Azure) on a ShP 2013 on line.

In my solution I have an aspx page (with a button) and a Client Web Part to add the page in ShP as an App Part.

All works fine but when I click on my button, doing a postback, I receive this error: "An error occurred while processing your request."

Debugging the solution, I see that on Page_PreInit the function SharePointContextProvider.CheckRedirectionStatus(Context, out redirectUrl) returns RedirectionStatus.CanNotRedirect

But I don't understand why...