I don't use portable version as my main browser in Opera and it installed on system partition. Also I always update my windows 10 version to latest one when updates available.I couldn't find a pattern for when this happen. to me it is few times happened. But i didn't get that notification repeatedly when visiting specific site. Notification ask for firewall exception for private and public network. when it appear i don't change my settings. Just apply with same it settings. Which is allow only for private networks.