Well... I didn't installed anything new for a while now...so hard to say, it just randomly started to pop up this issue. I will probably check that system file checker and if it wouldn't help then the only option would probably be to reinstall OS.

It is either forged (what I do not consider likely, although it is possible) or your Windows cryptography engine is somehow broken.

Yes, that was my thought aswell, and that's why I checked my pc for any hint about that winrootca on both my pc and laptop and on internet aswell, but couldn't find anything.

I didn't pay enough attention to this earlier. First, IE cannot be uninstalled from windows efficiently, but it can be hidden and rendered unworkable.

Well, yeah, I didn't actually uninstalled it, but isolated it and I did that ages ago, so i can hardly remember what programs I used... probably some guide on the internet.

What about Firefox - IIRC it doesn't use Windows internal crypto engine, at least not in full extent. IE/Opera/Chrome do.

Yeah, as i said already, FF works absolutely fine for me.

Russian is not problem What is visible on second tab (Состав) of O40 (get.adobe.com) cert? Путь сертификации (Certification path) is probably empty?

Oh, forgot about those. Here they are:

https://s18.postimg.org/4gsl2tswp/opera40_3.jpg
https://s16.postimg.org/3rojyfkud/opera40_4.jpg
https://s17.postimg.org/rslndxe7j/opera40_2.jpg

I forgot to ask - does your Internet connection use proxy? Internet options, Connection, LAN Settings - what options are there selected?

If i understand you correctly, then no, i'm not using any proxy.
That's what you're talking about, right?
https://s22.postimg.org/g6y71tnyp/opera_proxy.jpg

That's opera 12.18 issues. Even if i install english version it would still be in russian for some reason...
I just hope it's still gona be enough to understand it.

https://s16.postimg.org/vmh13g5xh/opera1.jpg
https://s17.postimg.org/eokfz5ltr/opera2.jpg
https://s13.postimg.org/dqmbmzpd3/opera3.jpg
https://s15.postimg.org/eq9ookzl7/opera4.jpg

And that's opera 40.0 one.

https://s16.postimg.org/nl27gqt79/opera40_1.jpg

Do other browsers, esp Chrome and IE, behave better?

Firefox works absolutely fine. Chrome has the same private connection issues.
I uninstalled IE, so can't tell about that one.

Scan your machine with different (online) antivirus/antimalware utilities (I can recommend ADWCleaner), maybe some of these reveals something.

Scanned, cured, still the same.

Can you post some untrusted server certificate data in detail (including certification chain or even export certificate and link somewhere)? Looking at such certificate it would be quite clear, is it real or inserted by some rogue process.

Will do that after i come back from work today

Have you checked with a clean separated(standalone) installation or with a clean profile?

If i understand you correct then yes, i checked it with a clean installation. I uninstalled all operas i had and cleared all files related ti them - cookies, register, etc. And still got this issue.

Do you use any extensions? What about Anti-virus or similar software?

The only one i used was Adblock, but i uninstalled it in one of the first tries to figure out this problems.
About anti-virus, i'm using Baidu Antivirus 2015.

Try updating to 12.18 as it was released to address issues regarding secure connections.

Well... in version 12 i had an issue with certificates only, it will spam me with any possible thing, like google.doubleclick, img0000_something, etc

It happens on that version too?

Yes, unfortunately. But not on all sites. Some will work, some will have private connection issues, some will have certificate issues.

When you try to go to a page and get the error, what info do you get if you click on the globe at the address bar?

If I try to open facebook i will get a private connection problem and globe would have next text:

Unprotected connection
www.facebook.com
The connection is not protected.

First visited: Saturday, October 1, 2016
Certificate: *.facebook.com

Connection: TLS 1.2 AES_128_GCM ECDHE_RSA (P-256)
Errors: Server's certificate is not trusted.
Server's certificate cannot be checked.

Windows 7 SP1
Opera 12.17 and Opera 40.0

So, here is a thing, around a week ago for some unknown for me reason opera started to spam "invalid certificate" on almost every site (including the ones like google, wiki etc). I tried to clear cookies, reinstalling opera, but i still getting this messages, even though now not on all the sites. On top of that now some sites are not accessible because of "Your connection is not private" problem...

The CA that is not seems to be reliable named WinRootCA. I couldn't find anything about it in the internet neither any files on my pc with some similar naming. I tried to install opera on my laptop and it's working absolutely fine.

How can i fix this thing? Both private connection and certificate one.