@leocg I found out that the SaveFrom's extension caused that problem. Already disabled it and thereby solved my problem. Anyway, thanks for the hint

While opened, Opera tries to open the gatpsstat.com website every 5 - 15 seconds. When I close Opera, it doesn't try to open that site. I scanned my PC with Malwarebytes, it detected some malwares. I removed all of them. Then I rebooted my PC, but it didn't solve the problem. How to fix this?