It looks as if I am stuck with FireFox in my Windows 7. This all works perfectly with Spam Arrest and Kasperky.

Ok, but if you want to get to the bottom of things, you'll need to disable those (one at a time) to see if one is causing the issue with Opera.

I assume your time, date, time zone and daylight savings settings are correct. If not, fix them (even if things work fine in Firefox).

In the Control Panel, goto "Internet Options". On the "connections" tab, click "LAN" settings. Does anything (like SpamArrest) have a proxy turned on? If so, disable the proxy and disable "automatically detect settings". to see if that makes a difference.

In your Network Settings under Network Connections, right-click on your ethernet or wifi adapter and goto "properties". Then, check the properties of your TCP/IP connections (both 4 and 6) and make sure IP and DNS are set to automatic (unless you know they're supposed to be set to something specific).

Look at the file "C:\Windows\System32\drivers\etc\hosts" in Notepad and make sure there are no active entries (non-blank lines without # at the start). If there are, you need to fix that, save and restart Windows.

Do you have Bitdefender and or Sandboxie on your system too?

Also note that things working fine in Firefox might not mean too much. It uses its own network stack and might not be using the system's proxy settings. You can check the proxy part in "Options -> General -> Network Settings -> Settings" in Firefox to see. But, even if Firefox isn't set to "No Proxy", it is still using its own network stack instead of the one Windows uses. You need to compare with other Chromium-based browsers like Chrome, Chromium, Vivaldi and Brave that use the system's network stack like Opera does.

Also, in Opera, download the Opera installer, launch it, click "options" set "install path" to a folder on your desktop, set "install for" to "standalone installation" and install. In that Opera, don't enable Opera Sync or VPN and don't import anything. Test in that Opera. If it works fine there, it means something is messed up in your normal Opera profile. If it still doesn't work there, goto the URL opera://extensions in that Opera and make sure there are no extensions (like ones for Spam Arrest etc.) that got installed on you.

Another thing you can try is to temporarily create a new user account on Windows, log into it and test Opera there to see if there's a difference.

Some testing to do, but it should help narrow things down. It's definitely something interfering with Opera.

there's something else going on such as malware, etc.

Yes, I have Spam Arrest installed - invaluable !
It looks as if I am stuck with FireFox in my Windows 7. This all works perfectly with Spam Arrest and Kasperky. I do miss the old Opera Bookmarking though.

Given that Spam Arrest employs POP proxy techniques, it may somehow be getting entangled in the process of vetting https certificates - which is what your original Security Issue message seems to refer to. If you've never installed Spam Arrest and you're getting this on all visited websites, there's something else going on such as malware, etc.

If so, try to disable all your security softwares.

This is brief compared to some of the other sites I have tried to access.

Are you getting that message when you access this forum?

In principle, there should be a button on one of the other tabs in that dialog to add the certificate to Opera's certificate store. But if the warning is a real issue you should fix the problem and not just ignore it.