Getting the same error on trying to preview any game including construct 3's sample games as well. Cleaned caches and cookies, allowed pop-up for preview under Site-Settings, restarted the browser and even system several times, scanned with anti-virus, updated java & also my Windows 10 (64 bit)... but nothing worked so far.
I am using construct 3 for the last 2 months now and for the past 2 days, I am facing this issue.