Internet Explorer 11 is not compatible with WinCC V7.1.

You can find the below table to know about compatibility of the WinCC/WebNavigator client with the Internet Explorer:

Notes:

• If you want to operate a WinCC/WebNavigator server and a WinCC/WebNavigator client on one computer, then they must both have the same version. This also applies if you are operating WinCC and WinCC/WebNavigator server or WinCC and WinCC/WebNavigator client on one computer.
• If the web client was connected first of all to the web server with the older version, then you have to delete the "CheckInstallation Class" file in "Internet Options / Temporary Internet Files / Settings / Display Objects" in the Internet Explorer of the web client. In isolated cases this is not enough and you have to reinstall the web client
• Pay attention to the compatibility of the WinCC/WebNavigator client and the WinCC/WebNavigator server with the Internet Explorer. Refer to the manual for each version to find out for which Internet Explorer the WinCC/WebNavigator client and server are released.
• Pay attention to settings that might be necessary in the Internet Explorer. Again refer to the version-specific manual.
  

I hope it helps you!