When you are trying to log in SAP Excel Add-in you are presented with a XML page with SSO Navigation Container window.
The SSO configuration has not been done or completed in the system and hence when trying to logon with the SSO the Navigation Container will redirect to the logon page of the system (which is the XML content that you see).
Please perform the configuration of the SSO environment in the system first.
Check Help Center documents:
Single Sign On
KBA , SRD-CC , Cross Components , Product Enhancement