Short: Try running steam as administrator.
Long: I tried the C2 free edition on steam (under windows 7) then upgraded to the personal edition. But I still got the same error, unable to start http server. I guess some other people have had that problem too.
What I found out was that steam was starting the free version and not the personal version. (Both versions were still installed). So what I did was this:
I hope this works for some of you experiencing this problem.
Develop games in your browser. Powerful, performant & highly capable.