[Security] Google tags Onstage Greenbelt page as "harmful"

I was searching for the Web site of Onstage Greenbelt via Google, and was surprised that Google thinks the site may harm my computer due to harmful Web content.

So entering "onstage greenbelt" yielded this search result. Note the warning after the page name:



Clicking on the page name directs you to this warning page:



So I manually entered the URL, and I noticed that the browser window was resized, as if I clicked the Restore button. While it is a sign of malicious activity, I have noticed no further evidence that there is something suspicious. I may be wrong, though.