"Unable to start HTTP server. You may need to restart Construct2 with admin privilages, or use disk mode."
It seems the local server provided for testing has some failure.
You can always go in the project properties and set "Preview mode" to "Disk" and check if it does work.
Some functions will be missing (such as audio which will work only in http).
Do you use some sort of local server to work on websites or something ?
Also, is it specific to a project in peculiar, or does preview fail on any project you make/open ?
(let's say you create a new project, leave it blank, try to preview it, does the error occur ?)