When you use Windows Server 2012 R2, Sentinel does not launch in Internet Explorer 11 due to the default security configurations of Internet Explorer 11. You must manually add Sentinel to the list of trusted sites before launching Sentinel.
To Add Sentinel to the List of Trusted Sites
Open Internet Explorer 11.
Click Settings icon > Internet Options > Security tab > Trusted Sites > Sites
Add Sentinel host to the list of trusted sites.