I have Endless Space in my Steam library, just started playing it recently, and was enjoying your game in the single player mode. Then, last night as I was shutting down my PC, I got a notice that Microsoft wanted to install patches. This morning, the game will no longer launch. It fails when it tries to great the full screen instance.

I looked up what the patch addresses, and it addresses vulnerabilities in Flash. My guess is that it is preventing you form legitimately doing some handshaking with the game client because the same route could also be exploited. Please address this dependency so the game game will run.

Running Windows 10, patches are up to date (which is the problem ...), worked fine before patch. Yours is not the only steam game affected, although not all are.