@leocg I reported it to Bitdefender yesterday. No reply so far.
I also reported it to the Opera security team: They were very quick in responding. This is what they say:
Sitecheck is our web service that warns users about malicious websites and there is no way it could infect your computer. It simply tells the browser to display a warning on chosen pages.
We believe it's a "false positive" alert and will contact Bitdefender about it.
Problem not yet solved...