Custom response on http status 401

Hello,

We upgraded our CAS server from 3.X.X to 5.X.X. With that upgrade our server send HTTP 401 with authentication is failed. The problem is that on environnement with a BIG-IP behind, user has not the page with message telling him the credentials was wrong but a almost balnk page with "HTTP ERROR 401".

I can't find where this behaviour is configured. I'd like to let the response pass to display the page to the user.

Thank for your help.