It is a known issue caused by a recent change in Intel graphic driver 10.18.14.x or later version.
If you are using Intel HD graphics (we could reproduce the issue at HD 4000 and 5000 series), and just update your driver to 10.18.14.x or later version, it will not be able to create a virtual VGA display anymore, so the Splashtop Extend Display product will not work.
We are working with Intel to resolve the issue.
Driver before 10.18.14.x:
Driver 10.18.14.x or later: