danteafg: Thanks for following this up. However, the (translated) reply from Bitdefender does not make sense. Uninstall what? An integrated part of the Opera browser? I am hoping that the Opera security team will convince Bitdefender that it is a false positive (alert)..

Confirmed! I have not had any messages from Bitdefender for > 16 hours.
Problem solved.

danteafg: Thanks for following this up. However, the (translated) reply from Bitdefender does not make sense. Uninstall what? An integrated part of the Opera browser? I am hoping that the Opera security team will convince Bitdefender that it is a false positive (alert)..

@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...

My Bitdefender security is constantly reporting the following critical alert on any website I visit:

Infected web page detected 1 minutes ago
Feature: Online Threat Prevention
We blocked this dangerous page for your protection:
https://sitecheck.opera.com/api/v2/check
Threat name: Trojan.Agent.ETKZ

The Opera browser seems to work OK. I get the same on three different PCs.
When using Firefox or Chrome vis I do not get these alerts.
"sitecheck.opera.com/api/v2/check" looks to me like a feature of Opera.
Or is it a false positive in Bitdefender.
Any ideas?