Disabling "Enable Chromecast support" definitely does not help. The problem with "debug.log" continues. Even in the latest version (57.0.3098.110).

I noticed that on my computers Opera generates these files starting from version 57. I wrote about it some time ago. I even set up a new profile. Unfortunately these files are still being created.

When I open a local .htm / .html file, a debug.log file is created in the same directory. The problem started with version 57.0.3098.76. I noticed that a similar file appeared also after installing a new version of Opera in \Opera\57.0.3098.76 directory.

Na stronie pobierania jest napisane wyraźnie i oficjalnie (w końcu), że to już program bez przyszłości.
Czyli, generalnie rzecz biorąc, jeśli używać Opery, to w duecie z innym programem pocztowym?

The issue has already been resolved with the help of Bitdefender support. In Opera 52 and (probably) Chrome "Experimental QUIC protocol" is enabled by default, so Google uses QUIC instead of SSL/TLS. I just had to disable the experimental QUIC support to restore SSL page filtering on Google (and other sites with QUIC) using opera://flags.

Bitdefender Tech Support has informed me that "Search Advisor is not compatible with Opera browser". It's weird. In versions of opera earlier than 52, Search Advisor worked perfectly in all search engines. Now it stopped working only on Google search and only in Opera. In other browsers (Firefox, Edge) Search Advisor works perfectly with Google search. I think this behavior may be related to the fact that Bitdefender uses its own certitficate to scan SSL traffic. The difference is easily seen in the attached screenshots. In Opera this certificate (from Bitdefender) is not used only for Google.

For now, I informed Bitdefender. I think they have to adjust...

With this version (52.0.2871.30, x64) Site Advisor service from Bitdefender has stopped working with Google search engine. It's weird because it works with other search engines like Bing, Yahoo, etc.