https://localhost/OWA/auth.owa gives 400 Error

On all my Exchange 2013 servers Scom reports that the OWA.Proxy is unhealthy and thathttps://localhost/OWA/auth.owa gives an Error 400. But if I tryhttps://localhost/OWA/ everything works fine and we have no problems with OWA on all our clients. What could be the cause of that error 400 ?

Thanks,

Antonio