You are using an ADFS ( Active Directory Federation Service) and modified the URL into the html5 URL. While you are using the SSO URL the System opens in Silverlight.
Cloud for Customer
As long as the Client is not changed from Silverlight to HTML5 the system will use Silverlight as default UI (User Interface).
Go to the Key user settings.
Open the Company Settings.
Select Client and change the preferred Client to Html5.
Now using the SSO Url ie. https://myXXXXX-sso.sapbydesign.com will open the system in the desired html5 UI
SSO, Single sign on. , KBA , sso , single sign on , SRD-CC-SEC , Security , How To